Well, not exactly, because Chaos is not actually evil, so he would not turn into an enemy, just the big bat thing inside Vincent being out.

He will snap in year nine following the death of Sephiroth during the personality switch event. That will affect Chaos too and he'll go on a rampage. I've already planned with Break-mun that he will be the one to stop him (and dying int eh process to maximise angst for Vincent), but Shepard could be a part of the first wave of fight the giant bat.

The result is that Vincent will be extremely withdrawn after wards, because he'll think he's a danger to everyone he cares about.