Magic stat maxes out at 255. Once you equip a mastered Magic Plus to anyone, they're at that, if not damn close. Either way, once it gets that high, it hardly makes much of a difference.

Anyway, by "using materia" I mean healing. Most people that use Vincent talk about his vaunted magic skill, and use him as a healer. While he's under that limit, that means that your designated healer can't heal, which sucks. And if you take all worthwhile materia off of Vincent before a battle so that this won't harm yourself too much, not only will your other two characters seriously be overloaded(and probably have to unequip a few materia of their own), but Vincent will suck even more when he's not in a transformation.