Use him all throughout Nibel until he gets his lvl2 limit break. This shouldn't take too long as, when he uses his fire limit break, he'll beat all the enemies, so his count should be pretty close to where it needs to be when you get to Materia Keeper after exploring everywhere, too. Just a few more battles oughta do it. Set his limit level to 2, and then Vincent will not heal the Materia Keeper, and then you'll like him again.