The more I play Vergil the more I believe he is the best at 2nd spot. Rapid Slash assist is really good for corner mix ups, lock-down and for keep away the push-back from the assist is the best in the game. Spiral Swords DHC do a ton of damage and basically allow Vergil to function at his best... when he is hitting and mix up opponents while burning meter.
I totally agree that Vergil is best at 2nd, but I really disagree with almost everyone that rapid slash is his best assist. There is some serious start up, it is completely vulnerable, it pushes opponents away (bad for when I am rushing with Wesker), it sucks for combo continuation (opponents go up very high and cannot follow up reliably), and the cross-up factor is mostly lost since you just block away from the point character.
The "Orb assist" (ie judgement cut) on the other hand is godlike.
1) It is a safer than rapid slash. Vergil comes out and leaves very quickly.
2) The recharge time is super fast. It charges like 50% faster than slash, giving you the ability to use it more frequently.
3) It nullifies projectiles completely and stays on screen for a bit. ie - I have seen it nullify all 3 sent drones.
4) It can easily be used for assist relaunch combos mid-screen. Rapid slash is very hard to follow up.
5) It causes a lot of block-stun -- this is what makes it godlike in my opinion.
6) Bonus points -- it can be hard to see onscreen as the opponent is coming in.
On an incoming character I like to call the orb assist -- they get hit with the orb while in the air. You can set up a guessing game where you will tb behind them (wesker) or wave dash under/behind/front while they are in block-stun midair. I usually go for the behind them first -- seems to be the most effective.
