I think it mostly deals with how they act in combat. Vincent almost cast magic the same way as him just brings both arms up a few inches and drops down kind of like Sephiroth. It could also be since Sephiroth wasn't planned to be fully playable they probably had to get the animations from somewhere. Its much easier to use existing data and tweak it then make a new one, but this is pure speculation on my part because I don't know exactly what goes into making the game. So I think basically they comped out.