How can I regain control of Vincent once he transforms?

Vincent's ultimate is nice, but sometimes his normal attacks are nicer. Especially when his magic attack is absorbed by the enemy:

Vincent healed Materia Keeper back to full health about four times and I was stuck with him in this form for the entirety of the battle.

Can I snap Vincent out of his limit? Would a KO do the trick?

Solution 1:

It is not possible to get him out of this mode for the entire battle once he enters it. Even if he is KO'd. See here for more details, scroll down to the bottom for Vincent. Please note that this linked site has info on all the limit breaks for every character.....

Solution 2:

You'll have to knock him out to stop the limit break, the only other way the limit break ends is when the battle ends. You can heal him after the KO, the limit will be over and his meter resets.

You'll just have to make sure not to use the "wrong" limit breaks (Galian Beast has fire, Death Gigas has lighting, other two are non-elemental) on the wrong bosses.