DISCLAIMER: I do not own Devil May Cry or High School DXD.

Vergil walked calmly through the empty school, heading towards the clubroom where the so-called Occult Research Club was located. Vergil was interested in two members of the Gremory house. Rias Gremory interested him the most. The Power of Annihilation interested him on two levels. He wished to beat it and he wished to turn Rias Gremory's soul into a Devil Arm. The other he had heard of was the wielder of the Red Dragon Emperor's Boosted Gear. Vergil was very interested in turning that into a Devil Arm, though he had heard that its wielder was useless. He had also heard of a Priestess of Thunder and a warrior wielding Sword Birth, and a child with an ability to control time. The more Vergil heard about this group the more interested he became in claiming their souls as Devil Arms.

Rias sat in the clubroom in front of her group discussing a group luncheon, when suddenly everyone turned to regard a cruel-looking white haired man who appeared at the door.
"This room is no place for a proper fight. All of you, step outside with me. None of us can fight comfortably in a space like this."
Issei stood up violently. "Who the hell's this douche?"
Kiba stepped up closest to Vergil, having noticed Vergil's sword and considered it would be like fighting another knight. Xenovia stepped up behind Kiba, readying Durandal. Akeno stepped up next to Rias, while Koneko and Issei both stepped forward and Asia stepped back behind Rias. Vergil noticed every movement made, and so far the only ones he felt inclined to respect were the young boy and girl with the swords, and Vergil assumed one of them was probably the wielder of Sword Birth.
"I already told you; this is not the place for a fight. Step outside and face me properly or I'll just destroy the school with all of you in it."
Kiba stepped forward, bowing slightly. "Very well. We'll take it outside, and we'll show you why messing with us is a bad idea!"
Vergil smiled slightly, impressed by Kiba's formal approach.
"I'm starting to like you…"

Vergil stood across the field from the Club members, who listened as Rias quietly laid out her plans. Vergil was graciously waiting for them, so they had time to prepare.
Rias turned to each of her servants in turn. "Kiba, Koneko, Xenovia, I need you three to take the front lines and fight him directly. Akeno, you fly above and wait for a clear shot at Vergil. Asia, you stay close to me and be ready to heal anyone who needs it. Issei, you assist Kiba and Koneko where you can, and use your Boosted Gear if you can get a clear shot. We need you to charge that gear up. I'll step up and assist whoever needs it. Gasper, we need you to try freezing Vergil if you get a shot."

Vergil looked up at the sky, watching as magic circles started to deploy. He knew enough about portals to know the circles where harmless on their own, but could be used for several things. He turned his attention to his opponents, watching as they started to mobilize. He knew the sword wielders would come forward and attack, and he expected the red haired girl and the dark haired girl would keep their distance. The boy and the small girls were the unpredictable ones.

Vergil watched as Kiba started the fight, charging at Vergil with his holy devil sword at high speeds. Vergil saw every step and was more than fast enough to react. Vergil deflected Kiba's attack, preparing to retaliate by stabbing Kiba, when a blast of red energy hit him, sending him staggering off balance. He looked up to see Rias Gremory pointing her hands at him, having obviously been the only one in a position to save her knight. As Kiba recovered, Koneko and Xenovia stepped up next to him.
Koneko folded her arms. "It was a bad idea rushing him like that. It's lucky Lady Rias saved you."
Kiba nodded. "I know. But we can still take this guy!"

Vergil teleported away, turning his back to them and thinking. He was quite unimpressed with Kiba's performance, but Rias' attack had been quite painful. Vergil suspected he could have dodged Rias' attack if he had seen it coming, but he hadn't been looking.

Kiba, Xenovia and Koneko started to slowly approach. They now knew Vergil was faster than any of them, which made them nervous, but Rias' attack had visibly hurt him, and as a group they could still fight him. Kiba wasn't going to rush in on his own again. Issei held up his hand, turning it into the red dragon claw. "BOOST!"
Vergil observed this, and now knew Issei was the one with the Boosted Gear. Vergil fired a Judgement Cut, leaving Issei with a deep cut through his stomach, dropping to the ground. Asia jumped down and started to heal him, and Vergil started to fire another Judgement Cut, this one at Asia. Rias appeared next to Asia and moved her away from the cut, having seen the move Vergil used to fire it.
"Heal him and then stay close, Asia. Those attacks aren't projectiles. They come out of nowhere at you!"

Asia nodded, blushing as she attempted to hide how scared she was. Vergil then charged smoothly forward, slashing and firing spectral blades. Kiba and Xenovia both gave ground before him, defending themselves from the sword, while Rias deflected the swords with her shield magic. Vergil then suddenly sheathed Yamato and started punching Kiba, kicking him away. Xenovia stepped up on her own, but Vergil punched her in the stomach, knocking her back. Koneko took Xenovia's place exchanging punches and kicks with Vergil, but Vergil was too fast for her and she soon found herself on the receiving end of a barrage of fast strikes. Vergil kicked Koneko away, when suddenly a blast of lightning came down at him. Unfortunately he saw it and teleported away, avoiding the attack.

Vergil then jumped up, firing a barrage of Judgement Cuts at Rias to keep her on the ropes, while his jump put him face to face with an exposed Akeno. Akeno deployed a shield, but Vergil shattered it in a single strike and then slashed Akeno through the stomach. Akeno screamed and fell to the ground, blood pouring from her wound. Rias saw her best friend wounded as she was, and charged up into the air, firing blasts of red energy at Vergil, who blocked them all. Rias flew back, watching in horror as Vergil teleported back to the ground, looking up at her impassively.
"You might want to stay up there. It won't save you, but it makes it a bit harder."
Kiba suddenly jumped up behind Vergil, attacking from above, and gasped in horror as Vergil vanished, appearing behind Koneko and stabbing her in the back. Koneko cried out and dropped to the ground, and was soon picked up by Issei, who had also picked up Akeno. Issei quickly carried the two wounded girls towards Asia, when Vergil started firing spectral blades and Judgement Cuts at him. Rias appeared between Issei and his attacker, blocking the attacks with her shield, while Kiba attacked Vergil from the back, cutting Vergil across the face with his sword.

Xenovia closed in from the front, trapping Vergil between herself and Kiba, but Vergil teleported away from Kiba and Xenovia, landing smoothly on his feet, smirking as the cut on his face closed by itself.
"Don't you people know a one-sided fight when you see one?"
Asia started to heal Akeno, as she was more seriously wounded than Koneko. Vergil saw Asia healing the others. Such an ability was of no interest to him since there was nothing the healing could do for him that his own regeneration didn't. Therefore, killing the girl was a perfectly obvious strategy. Vergil fired a string of Judgement Cuts at Asia, who was cut in several places, and only survived at all because Issei caught her and dragged her away before the cuts all hit her.

Rias stepped in front of Vergil, glaring at him in rage and glowing red as her power flared angrily.
"I will destroy you, stray devil! In the name of the marquis of Gremory, and for what you've done to my servants!"
Rias started firing blasts at Vergil, but he teleported up to her and started casually jumping, avoiding the blasts with insulting ease. Kiba and Xenovia charged in beside Rias, attempting to hit Vergil but meeting with the same insulting lack of success. Vergil then vanished, appearing above Xenovia and slashing her deeply with Yamato. Vergil kicked Xenovia in the face, using her as a step to move further away. Xenovia gripped her wound briefly before dropping, exhausted, leaving Kiba the only remaining swordsman.

Issei cradled the wounded Asia in his hands. Akeno had been healed to the extent that her wound wasn't fatal, but neither she nor Koneko could continue to fight. Kiba appeared beside Issei, placing a wounded Xenovia next to Akeno and Koneko. Asia was also bleeding badly. Her wound wasn't quite fatal, but she needed help soon. Issei ground his teeth as he thought of the monster that had done all of this. As Issei fumed, his Boosted Gear lit up brightly.
"DRAGON BOOSTER!"
Snarling with rage, Issei turned around and fired a blast at Vergil, who was locking blades with Kiba while holding Rias by the neck with his other hand. The light and sound from the blast alerted Vergil to its presence, and he attempted to jump away, but Rias attacked with a renewed force, hoping to keep him grounded and send him towards Issei's blast. Vergil had already jumped but was still hit and launched back towards Issei's blast. Kiba charged in, slamming his Holy Devil Sword into Yamato to prevent Vergil from teleporting. Kiba felt the strike quite sharply, but succeeded in preventing Vergil's teleportation, allowing Rias' blast to carry Vergil to Issei's, leaving the stray devil caught directly between the two blasts.

The blasts exploded, launching Kiba back into Rias' arms. Issei slumped down, having put much of his life force into that blast. In the wake of the blast crouched Vergil, breathing hard and noticeably exhausted but still showing no injury. As Vergil started to move forward again, albeit much more slowly than before, he suddenly froze, a strange aura around him. Issei turned to see Gasper staring at Vergil to keep him frozen.
So Gasper's power works on this douchebag? That's good! He's weaker than Sir Zechs!
Issei charged, and found himself accompanied by Kiba and Rias. Issei's fist connected sharply with Vergil's face, directly followed by a bolt of Rias' red energy. Kiba then appeared in front of Vergil, slashing him straight across the chest, a strike which would split him in half when time resumed. Issei pumped his fist, kicking Vergil in the crotch as a sign of disrespect. Gasper released the time stop, and Vergil staggered back, snarling. Issei, Rias and Kiba were stunned. Kiba's strike should have split Vergil in half, and yet he was whole, and didn't even have a wound. Issei didn't understand, but Kiba and Rias both did.

Vergil snarled with rage, his scream changing into a deeper, demonic voice as he changed into a blue demon. Vergil vanished and appeared beside Gasper, slashing the young boy across the chest and leaving him badly wounded. Kiba ran up to Vergil, ready to put the Holy Devil Sword through his face this time, but gasped as Vergil vanished three times in a row and appeared next to him, sheathing the sword. Kiba cried out as multiple cuts appeared on his body, and collapsed, no longer able to fight. Rias fired another blast at Vergil while Issei charged at him, even angrier than before.
"Fuck you!"
"BOOST!"

Vergil dashed past Issei and slashed him through the stomach, but as soon as he stopped moving Issei was upon him, punching him with his Gear. Issei held up his hand, boosting again and charging at Vergil while Rias fired more blasts and advanced closer to help the last servant she had still standing.

Vergil faltered as Issei's enhanced state started hitting him, and Issei's heart leaped as Vergil reverted to his human form just in time to be hit in the back by one of Rias' attacks which sent him into Issei's fist. Issei then grabbed Vergil, holding out his Gear to Rias.
"President!"
Seeing what Issei wanted her to do, Rias fired a blast of her power into Issei's arm, which consumed it. "TRANSFER!"
Issei then slammed his fist into Vergil's face, releasing an explosion of red energy which sent Vergil skidding across the ground on his back. Vergil jumped back to his feet, but dropped to one knee again.
"Damn you!"
Issei charged, while Rias fired more of her blasts at Vergil, who deflected some but not all. Issei slammed his fist into Vergil, staggering him back. Vergil slashed at Issei, but Issei dodged the swing, jumping back. Issei and Rias both charged in for a joint attack, while Vergil activated his Devil Trigger one final time. Issei slammed into Vergil, but Vergil slashed his arm sideways and dashed past him, leaving a deep cut in his side. Vergil then jumped up into the air, avoiding Rias' blast and firing a barrage of Judgement Cuts numbering in the dozens and covering the entire field.

Issei was hit by several Judgement Cuts, and was now covered in deep cuts. Rias had been hit by one but then flown up higher to avoid them. She hated that she couldn't save Issei from getting hit, but she knew that losing to Vergil meant death for all of them. Rias cringed as Issei dropped to the ground, exhausted and bleeding from dozens of cuts. Vergil was then upon Issei, slashing him through the stomach and then kicking him onto his back. Issei was out of the fight now, and Rias was the only one left.

Vergil snarled as he reverted to his human form. His energy was running out, and had he been able to stay in Devil Trigger killing Rias would have been easy, but Vergil was almost finished himself. He turned to face Rias, who hovered above him, every bit as exhausted as he was.
"Let's finish this, Vergil! Messing with my servants is worthy of a thousand deaths, but messing with my Issei is even worse!"
Rias charged down at Vergil, firing blasts at him, but Vergil teleported up to her and started slashing her across the torso. Vergil slashed Rias through the stomach before slashing through her chest. He then readied to make the final strike which would cut her head from her body. As he was about to he decided not to and instead slashed her shoulder deeply. Vergil then teleported away and sheathed his sword, not looking as his cuts took effect on Rias' body. Her stomach suddenly had a deep cut in it with blood running from it, her large chest suddenly spurted blood from a cut across the top of her breasts, the blood rapidly turning her white shirt red, and finally her shoulder split open, cut deeply enough to expose bone and marrow.

Vergil turned to watch as Rias dropped to the ground with a groan, glaring at him as her eyes filled with tears. Vergil stepped over to her and glared down at her as she struggled to compress her wounds to stop the bleeding.
"I may come back for your souls one day, but I think you deserve to live after doing so well."
Rias relaxed slightly as Vergil left, and started to doze after about a minute, but then jumped as Asia appeared over her, healing her wounds. Asia had obviously healed herself and then started on the others. Issei was crouched behind her, and Akeno, Xenovia and Koneko looked less hurt too, though they were still down. Rias sighed with relief as she saw Kiba start to stir slightly, and Gasper was licking the blood from Issei's arm, which had deep cuts on it. Rias was furious that her servants had been hurt so badly, and humiliated at the fact her entire family had been defeated by one opponent, but she was glad everyone was alive.

Vergil sat down in a back alleyway, sombrely thinking about his decision to spare the Gremory's. He didn't know why he had chosen to cut into Rias' shoulder when he had been perfectly positioned to cut off her head, and he didn't know why it hadn't occurred to him to completely butcher the girl with the healing power rather than just injure her. Vergil usually didn't hesitate to kill people, and he prided himself on leaving his enemies as dismembered corpses, yet for some reason he had chosen not to do the same to the Gremory group.
"My humanity… It's pathetic! I… I must obtain more power! I must abandon this cursed, weak human side! I will surpass my father! I will defeat the Prince of Darkness like he did! I… need… more… power!"

Analysis:

This was a close fight. Obviously no individual member of the Gremory household can match Vergil. He is faster and more skilled than Kiba, stronger and tougher than Koneko and more powerful than Rias. However, the group as a whole can match him quite well. A combination of Rias' attacks and Akeno's could damage him quite badly, and while it is true he could easily react to them, with Kiba and Xenovia fighting him and the attacks coming from behind he can be hit, as Kiba is easily superhuman with his speed. Issei's blast is an attack that can do more damage, but it still won't stop Vergil. Asia's healing ability enables the group to somewhat close the gap made by Vergil's regeneration.

All in all, as a group they can hit him, defend against him and damage him. Gasper's time warping ability is one thing that will likely work on Vergil, since while Dante and Mundus and demons like that are immune to it, and as Nelo Angelo Vergil was immune to it as well, Vergil was vulnerable to Quicksilver in DMC3. However, Gasper has trouble aiming his time control, so chances are most of the group will go down before he hits Vergil with it. Either way, Gasper rarely keeps time stopped for very long, and with the exception of Ddraig, nothing the Occult Research Club have can take Vergil down in that time.

However, the obvious flaw in this plan is that several members of the household are necessary and yet can be taken down quite quickly, such as Asia and Akeno. Koneko isn't fast enough to defend herself against a full speed attack from Vergil, and thus she will go down easily too. Xenovia isn't all that much faster, so she too will go down. The only members who can properly hold Vergil off are Rias due to her power, Kiba due to his speed and Issei due to his tenacity, but none of them can really match Vergil and with their friends getting picked off it's only a matter of time before their numbers dwindle.

At the end Kiba, Rias and Issei were the only ones standing, and with Kiba going down due to Vergil focusing on him next as he is the next weakest link, it left only Rias and Issei, leaving them with no one who could fight Vergil's speed. At this point Issei is likely to go down next since he has a tendency to charge in, and between Vergil's rapid recovery and regeneration Issei is the one who will be outlasted despite his tenacity.

That left Rias, who has the power to counter Vergil's but not the speed to fight him, so on her own there was nothing to stop Vergil from cutting her repeatedly, and she cannot regenerate from severe wounds, and from a strictly engineering standpoint her slim build, thin neck and large breasts make Vergil's job of cutting her up even easier since her thin waist makes her organs easier to reach, her big breasts are easy to hit for obvious reasons and will bleed badly, and her thin neck would be easy to cut through, the same with her thin arms, all in all making her very vulnerable to Vergil's sword.

In short, the group's ability to fight Vergil lessened greatly as its members were brought down, while Vergil can keep going and can use his Devil Trigger if he needs to bring down someone particularly difficult or increase his strength to counter-balance his lessening stamina.

While it is true that if Issei had access to his armoured state he would best Vergil with it since he matched Vali Lucifer who was more powerful than Vergil, Issei can't use the armour without sacrificing his arm. Issei wasn't prepared and thus didn't have the armband Azazel gave him.