DISCLAIMER: I do not own Bleach or Devil May Cry. Bleach is the intellectual property of Tite Kubo and Devil May Cry is the property of Capcom.

Byakuya Kuchiki stepped into the captains' meeting, taking his place between Retsu Unohana and Shunsui Kyoraku. The head captain Shigekuni Genryusai Yamamoto tapped his walking stick on the wooden floor, sending out a resounding clacking sound.
"This meeting will now be called to order! Soi Fon, report!"
The small woman that was the captain of Squad 2 and the Stealth Force held up a clipboard and read from it.
"There are many disturbances happening in the world of the living, head captain, in many places. We first believed it to be the work of hollows, but we have caught one of the perpetrators. It should be in Captain Kurotsuchi's lab right now."

The monstrous scientist that was Captain Mayuri Kurotsuchi of Squad 12 held up a set of statistics, and read through them.
"Well, the creature's body showed incredibly low levels of brain activity, even lower than most hollows. We have compared its intellect to most animals and believe it to have intellectual capacity comparable to that of an alligator or crocodile. In short it is really just a killing machine, and containing it is no easy task. It killed two members of my squad yesterday by firing its claws at them. Interestingly enough, it displayed a regenerative capability similar to some species of shark or crocodilian, namely that its claws regrew instantly from their sockets. I already dissected the expended claws and found its claws are harder than any beast's claws, more reminiscent of the material that composes a creature's teeth. Problem is, I doubt such an obviously engineered killing machine is the result of mere evolution, but rather of scientific research and creation."

The captains showed slight reactions at this, and Mayuri continued.

"It so obviously combines many natural defences and abilities ranging from the way it regrows its claws like a lizard regrows its tail, the way it digs under the ground like certain insects do, the way it is actually capable of propelling itself through the air almost like a bird, swimming underwater as flawlessly as a fish, and even breathing both underwater and above water, something evolution still hasn't created. This is no mere animal, at least not a natural one. Someone attempted to create a life form with capabilities beyond anything nature has, and they apparently succeeded."

The captains exchanged glances, and Shunsui asked, "And do you believe that this creature is the pinnacle of their research?"
Mayuri held up his finger, smiling maniacally at Shunsui. "I was hoping someone would be smart enough to ask that. After all, if that creature is the best this person created, we are merely dealing with a set of creatures that are dangerous, but are really not that different from hollows at the end of the day. Unfortunately, I do not believe that it is the pinnacle. Not even close. For one thing, this creature may have numerous defences reminiscent of animals, but its DNA and chemistry show none of these creatures are a part of it. In fact, it is not related to any species of animal on this planet, past or present, even though it clearly bears resemblance to many of them. In fact, its genetic codes are effectively blank. It more resembles a robot than an organism."

"What does that mean?" Asked Shunsui, noticeably the only one in the room apart from Unohana who didn't wear an expression of absolute ignorance of Mayuri's meaning. Mayuri had always regarded Shunsui as one of the less stupid captains.
"Well, Captain Kyoraku, it means this organism was not created in a lab from gene splicing, from a dead body that was modified, or even a clone of various creatures. If it didn't have blood and brain activity I'd believe it was a robot. This creature appears to have been created, not in a lab from specimens, but out of nothing. It has no semblance to any life form except its appearance and defences, not even any meaningful DNA. This creature was created using a process I have no familiarity with, and if I believed in higher beings I would say we were dealing with one."

Shunsui narrowed his eyes. "Do you think these creatures resemble Nero in any way? The blood I got off him?"
Mayuri shrugged, sounding bored. "That boy had interesting blood, showing many strange things about his physiology, including similarly abstract DNA, but if this creature had borne any more resemblance to him other than the fact they both defy classification, I would have mentioned it."

As the captains continued speaking, a messenger knocked on the door, entering at the head captain's request and bowing. "My sincerest apologies, captains, but this request has come from Lieutenant Matsumoto and Lieutenant Abarai, and they say there is news from the world of the living concerning these creatures."
The messenger handed a piece of paper to the head captain, vanishing when the head captain waved him off. Captain Yamamoto read the paper, and turned to the captains.
"A man has appeared in the world of the living, and he has attacked both these creatures and our operatives. Apparently he defeated Lieutenant Abarai before the lieutenant could request a Gentei Kaijo or even release his Bankai, and has left piles of these creatures and others like them dead."

The tall form of the serious Squad 6 Captain Byakuya Kuchiki stepped forward. "My lieutenant being defeated so contemptuously by an unknown makes this my squad's affair. If you are willing, head captain, I request you send me to the world of the living to apprehend this person."
Head Captain Yamamoto considered this, and nodded. "Very well. I give you clearance to go to the world of the living, without a Gentei Rein, but I refuse to entrust this entirely to your squad. Captain Hitsuguya shall accompany you."

The short white-haired Squad 10 Captain Toshiro Hitsuguya stepped forward, looking less reserved than Byakuya. "Of course, head captain."

Vergil caught a Frost by its neck, turning briefly to regard the soul reaper who had instantly frozen to death just by coming within striking distance of this creature. With a huff of contempt, Vergil squeezed the creature's neck hard enough to shatter the ice, and then punched it with his other hand, smashing the demon into hundreds of pieces. Vergil then held up his hand, noticing the layer of ice that had formed over his hand, the result of the Frost's cold having frozen the moisture from the air. Vergil smelled something behind him, and turned to swipe Yamato through a group of Assaults and soul reapers, killing all of them.

As he started to leave, a door appeared out of thin air in front of him, opening to allow the exit of two humanoid beings in robes similar to the soul reapers except with a white robe over the black one. Vergil only knew about soul reapers because he had studied them, but he was still smart enough to know the white robe was some sort of an indicator for rank. The two beings advanced, and Vergil rapidly took stock of them both. The shorter, younger one seemed like he knew how to fight but he was obviously a more emotional type who would get lost in his own anger. He seemed concerned for the soul reapers that had come before him. His softness made Vergil want to kill him right there.

It was the other one that interested Vergil. He wore a serious, impartial expression that screamed judgement and purpose. He also walked smoothly, his serious brown eyes locked onto Vergil's own blue ones like a cat staring down its prey. Vergil smirked slightly, his eyes lighting up as he recognised a challenge. As Byakuya and Vergil stared each other down, a weak female voice called, "Captain…"

Toshiro turned to see Rangiku Matsumoto leaning against a building, limping over to him. She had deep blade wounds in her body, which a group of demons had inflicted with a little bit of help from Vergil. Toshiro flash stepped over to Rangiku and caught her, inspecting the wounds.
"I need to get her out of here, Captain Kuchiki. We should also find all the other survivors."
Byakuya flash stepped to Toshiro's side, placing himself between Toshiro and Vergil just in time to block a spectral blue sword that flew at Toshiro.
"Very well. Get out of here. I doubt you would last in a fight between him and me anyway. Not as you are."

Toshiro ground his teeth at this insult, until he decided Byakuya meant his concern for Rangiku. Nodding, Toshiro carried Rangiku to a safe distance, finding several members of Squad 4 already helping several wounded. Lieutenant Isane Kotetsu gave Toshiro a look of concern as she took Rangiku from him. Toshiro looked back at where Byakuya and the man stood, and saw that they still hadn't moved, and were staring silently at each other. Toshiro shuddered, realising just how quickly either of those two could end his life were he in the middle of them. With that, Toshiro decided to help Squad 4 treat the wounded.

Byakuya looked at Vergil passively, while Vergil matched his attitude.
"You are fast, boy, but you will find that speed on which you are relying is insufficient to save you."
Vergil used his thumb to lever Yamato slightly out of its scabbard, and in a flash he flew past Byakuya and slashed at him repeatedly, while Byakuya blocked the slashes. Vergil dashed back, shooting more spectral blades at Byakuya, but Byakuya slashed the blades from the air, retaliating by dashing back at Vergil, the two circling each other at speeds none of the Squad 4 members were able to follow. The two clashed at that speed, slashing at each other at massive speed and finally launching each other back, both charging immediately after.

The two traded blows, and Vergil shoved Byakuya off balance, displaying slightly greater physical strength. Byakuya flash stepped away, pointing his finger at Vergil. "Byakurai."
A beam of energy fired from Byakuya's finger at Vergil's chest, but Vergil sidestepped this shot, charging again. Byakuya and Vergil traded several more blows until Vergil jumped back, sheathing his sword and firing several Judgement Cuts at Byakuya, which Byakuya dodged. Jumping up into the air, Byakuya held up his sword. "Scatter, Senbonzakura."

The sword faded into a flurry of petals, which flew at Vergil. Vergil dodged the attack by jumping away from the petals, and fired a Judgement Cut at Byakuya, who dodged it and directed his petal blades at Vergil, who rapidly spun his sword, deflecting the petals. Seeing this, Byakuya reformed his sword and charged, attempting to stab Vergil, who for his part drew Yamato and prepared to stab Byakuya in retaliation, making no effort to save himself from being stabbed. The two collided, Vergil taking Byakuya's sword through the heart while Byakuya took Vergil's sword through the lung.

The two glared at each other, Byakuya waiting for Vergil to fall, while Vergil leered at him with a look of confidence. Both pulled back, Vergil's wound spraying out a small spray of blood while Byakuya's sprayed blood all over the ground. Byakuya leaned forward, clutching his wound and gasping, while Vergil casually sheathed his sword, and charged, attempting to vivisect Byakuya while he was wounded. Byakuya reacted surprisingly well, catching Yamato on Senbonzakura and slamming his heels into the ground to keep himself in place. Byakuya slashed again, and Vergil vanished, appearing several paces away.

Byakuya intensely examined where he had stabbed Vergil, noticing that the wound wasn't even there anymore. Deciding he needed to step up his game, Byakuya turned his sword to point downwards and let go of it, saying, "Bankai. Senbonzakura Kageyoshi."
The blade sank into the ground as if into water, and dozens of large blades appeared from the ground behind Byakuya, all bursting into pink petal blades. Seeing this attack coming, Vergil charged, firing several Judgement Cuts into the swirl of petals and launching himself at Byakuya. The swirl of petals enveloped Vergil, launching him away and cutting him repeatedly. Vergil vanished from the swirl of petals, appearing in front of Byakuya and slashing at him, but more petal blades got between them and blocked the strike.

Vergil ran back, outpacing the petal blades if only barely. Seeing this, Byakuya held out his hands, directing the petal blades. The petals instantly sped up, overtaking Vergil and cutting him repeatedly, also drawing him into a larger swirl of petals. Vergil snarled with anger as the petals cut him from all sides, his blade slashing away many but not all. Byakuya continued directing the petals with his hands, watching the petals repeatedly cut Vergil.
"Your final breath will be coming soon. When you die you will be instantly forgotten and disregarded as a particularly strong hollow. That is if you don't end up on Captain Kurotsuchi's slab being experimented on."

Suddenly, a burst of blue energy came from inside the swirl of petals, and the petals were forced away. Vergil stood in their wake, blood covering the blood under him, but now looking like a blue demon of some sort. Byakuya nodded. "A resurreccion, is it?"

Byakuya gestured, and all the petals withdrew, turning into glowing pink swords identical to his own original one. "Senkei, Senbonzakura Kageyoshi."
Vergil admired this display briefly, before launching himself at Byakuya with a demonic scream. Byakuya blocked the first set of blows with one of the swords, trading several blows with Vergil before holding up his hand and calling another sword into his other hand, stabbing that blade into Vergil's heart. Byakuya then rapidly stabbed his first sword into Vergil's stomach, and called two more swords, stabbing them into Vergil's feet. Vergil snarled aggressively, and Byakuya gripped his arm, holding back the sword Vergil still carried. "This fight is over."

Byakuya called two final swords and slashed Vergil through the neck with both of them, a move which would decapitate any soul reaper. To Byakuya's shock, Vergil's neck sealed itself up as he slashed it, leaving Vergil's head still firmly attached. Vergil suddenly vanished, the swords that had been in his torso vanishing with him while the swords that had been through his feet stayed in the ground. A barrage of Judgement Cuts appeared all throughout Byakuya's circle of swords, smashing several of the swords and cutting the trapped Byakuya repeatedly. Byakuya felt his robe and body shredded by one slash after another, until finally the demonic monster that was Vergil reappeared in front of him and flew past him, slashing him repeatedly in multiple places.

The exhausted Byakuya dropped to one knee, and Vergil reverted to his human form, sagging briefly as his strength lessened. Byakuya dragged himself back to his feet and faced Vergil, while Vergil prepared for their final exchange. A burst of white energy appeared over Byakuya as he prepared to charge, the floating swords all merging into a single sword as he focused everything he had into a final blow. Vergil gripped Yamato, changing back into his Devil Trigger and charging, while Byakuya charged also. The two slammed into each other, shooting out an explosion of dark blue and light blue energy, which blasted several buildings flying and terrified the squad members who were still not very far away.

In the wake of the blast, Vergil and Byakuya stood facing each other, Vergil back in his human form, each sticking his sword through the other's chest, and each covered in blood from the blast of energy and blades tearing into them. After a few seconds, Vergil stepped back, forcing Byakuya's sword out of his chest and gripping its blade tightly with his hand. He then leered at the weakened Byakuya and roughly extracted his own sword from Byakuya's chest, pushing Byakuya over and rapidly pulling the Senbonzakura from Byakuya's weakened grip, claiming the sword for himself. Vergil rapidly left, easily outrunning the squad members who tried to pursue him.

Analysis:

Vergil and Byakuya are very similar. Both rely primarily on speed and sword strikes, as well as long range blade attacks.

Speed:
Vergil and Byakuya both move at roughly Mach 10, and Byakuya's Bankai is twice that speed, but Vergil has two things with which to counter that. Overall, Vergil has the advantage in speed for two reasons, namely that he can teleport and the feats in DMC3 where Dante can dodge and block lasers from Damned Rooks and blasts of light from Beowulf the Lightbeast, accompanied by the fact Vergil can block shots from Artemis which behave like lasers and are repeatedly stated to be lasers in their descriptions. Those feats require Vergil to possess relativistic to light speed reactions in addition to his own speed.

While obviously inferior to actually moving at light speed, combining light speed reactions with teleportation, even short range teleportation like Vergil's, makes a combatant very hard to hit unless their opponent can counter their reactions, be it by being fast enough to hit them regardless like Mundus was or simply attack until his lower movement speed prevents him dodging which is how Dante and Vergil fight, or by something like time control like Barragan's Arragante or Dante's Quicksilver or Bangle of Time, or sensory manipulation like Aizen's Kyoka Suigetsu, though Vergil's ability to cut through illusions may make that one a bit uncertain. In short, Vergil has the advantage in speed.

Strength:
Byakuya can be safely assessed to be capable of lifting in excess of 50 tons, since Ichigo and he were of comparable strength when Ichigo competed with Kenpachi, who was able to physically wrestle Yammy's giant building-sized form, though we should consider Kenpachi was never physically weaker than Ichigo during their fight.

Like Byakuya, Vergil has no strength feats of his own except punching Beowulf to the ceiling, which in itself is at least a ten ton feat, but Nero has a feat of blocking Berial's sword thrust with the tip of his own sword then flicking Berial's sword away, all with no apparent effort, and Dante has a feat of casually blocking the Saviour's punch. These feats are both more impressive than Vergil's, to different degrees. Nero's blocking Berial's thrust would also be at least ten tons, more when you consider the shockwave that resulted from it, and casually flicking the sword back makes it more impressive.

Dante blocking the Saviour's punch is much higher, seeing as the Saviour was a skyscraper-sized demon made of stone both stated and then confirmed to be bulletproof, swinging its fist down at supersonic speeds, and he casually blocked it. That showed physical strength greater than any Bleach feats, but that was Dante in a far stronger incarnation set after the fight with Nelo Angelo, Abigail and Mundus, an incarnation of Dante which could frankly crush Vergil like a cockroach.

With that in mind, assessing Vergil's strength is hard, but obviously assuming it's equal to DMC4 Dante's would be like assuming Byakuya's is equal to Post-Dangai Ichigo's. This makes Dante's feat an indicator, but nothing to scale by. Vergil may well possess million ton strength, but since Dante overcame so many powerful enemies and improved so much while Vergil only has his DMC3 incarnation, it is far from solid. Nero is a more accurate measurement, but his Devil Bringer shouldn't be included there since it is a weapon totally focused on crushing power, and just because Dante is stronger than it doesn't mean Vergil is. Overall, Vergil's strength could be anywhere between ten and a million tons, since while Dante and Nero are both closer to the latter, both are stronger than him physically. However, the fact he can casually smash demon stone which is much stronger than the regular kind seeing as spindly statues made of it can be shoved up from under the street on top of Temen-Ni-Gru with no damage at all, indicates he's probably around the one hundred tons mark, probably higher. Granted the one seen smashing demonic stone is also Dante rather than Vergil, but they are at least similar, and Beowulf did something similar.

Skills:
Both of them have a tendency to believe in their own superiority simply for being born what they are, as shown by Vergil's disregard for the worth of human strength and his belief that Arkham's alleged human emotions made him weak, as well as Byakuya's statements that referred to hollows as not possibly his level, and humans as inferior, and finally, all those of "common" birth being inferior, as shown by his statement about Bankai being effectively the property of the rich families and no one else has a right to achieve it. However, while both are arrogant, Byakuya's arrogance is worse. Vergil instantly acknowledges when Dante achieves his Devil Trigger, though admittedly Vergil knew Dante was his twin brother and thus knew Dante was no joke, whereas to Byakuya Ichigo was a stranger.

Regardless, Byakuya is worse. Byakuya often wilfully ignores the feats of those he deems beneath him, seeing as how he completely disregarded Ichigo's growth rate and refused to accept that Ichigo had achieved Bankai. In all seriousness, less than a month passed between Ichigo and Byakuya's first fight when Ichigo lost Rukia, and their second fight on the walkway. When they first fought Ichigo was so much weaker than Byakuya that even when Byakuya was wearing a Gentei Rein, Ichigo was not only unable to see his slashes, but wasn't even able to feel them until after Byakuya finished. When they fought on the walkway, less than a month after that, Ichigo was fast enough to follow every step Byakuya took and fight him evenly, despite the fact Byakuya was no longer wearing a Rein and Ichigo was wounded after fighting Kenpachi.

To put it in a simple equation, he went from being crushed like a cockroach by Byakuya at a fifth his usual power, to being able to properly engage an unrestricted base Byakuya while wounded, and this development took less than a month. Ichigo is fifteen at the time. Anyone who wasn't dangerously overconfident would be worried about a growth rate like that, especially since being fifteen means he can surely get stronger. Even the infamously overconfident Frieza from Dragonball admits being worried about the power boosts the Saiyans have, namely Vegeta, since Vegeta goes from weaker than Cui to stronger than Dodoria but weaker than Zarbon. He then gets stronger again and kills most of the Ginyu Force, and while Frieza is dangerously overconfident, Vegeta's alarming growth rate does bother him, since any common sense would say that if Vegeta's growth continued at that rate, which it does, Frieza will be under threat, which he is.

That same logic applies for Ichigo. An enemy who displays as rapid a growth rate as Ichigo did would be cause for concern, as he has gone from lower than a fourth seat as he barely beat Gentei Reined Shikai Renji who was demonstrated to be vastly weaker than Ikkaku on account of that same Rein, even when he had a Bankai, and Ichigo went from there to a captain class able to face Byakuya in less than a month, a growth rate most Soul Reapers would kill for, and yet Byakuya actively dismissed Ichigo's growth rate, something everyone else noticed, be they Kenpachi, Renji, Rukia, Ikkaku, Ukitake, Ulquiorra, Grimmjow, Aizen, Gin, etc.

In short, Byakuya is deliberately closed-minded and wilfully ignorant in relation to anyone's capabilities if he doesn't want to believe they could be worth anything. Unless he's heard of Sparda, he would likely dismiss Vergil as a lower Soul Reaper pretending to be noble or maybe even an Arrancar, and thus he would actively ignore many of Vergil's abilities. If he's heard of Sparda he will respect Vergil, since Sparda would be respected among Soul Reapers as they value warriors, but otherwise Byakuya would think of Vergil as no match for him, and would likely be oblivious to Vergil's reaction speed also, whereas someone like Shunsui would probably notice Vergil's dodging patterns and work out his reactions were faster than his actual movement.

Durability:
This was probably the main issue. While both can take the energy blasts and have similar durability in that sense, Vergil is at a big advantage in terms of surviving being slashed and stabbed. While Byakuya has survived being stabbed numerous times, as do other soul reapers, Vergil can survive worse injuries and has regeneration to go with it, making him almost impossible to kill with Byakuya's blade attacks. That gave Vergil a clear advantage.