I saw this formula on another site:

Damage = (Strength * 10) + (Level * 10) + (enemies killed)

But then the attack stat of every weapon is also added to strength, is it not? And the Death Penalty has a 99 Atk stat, so that gives you a good start.

I don't see how level really fits into the formula because your strength stat increases as you level up, but I'm not gonna dispute it because I hardly used Vincent myself and so the Death Penalty was pathetic for me.

It gave the example of Vincent at level 75 with a 62 Strength stat having killed 500 enemies should do ~2000HP damage per turn. So you'll need to get his strength up a bit, and you've got a LOT of enemies to kill if this actually is the right formula. If you've been using Vincent throughout the game then it shouldn't be too big a problem, but if not it could take a while. =\