If you just delete the script(main) on Vincent, you should make him controllable. You can do this with Wallmarket rather than hex editing. Now, what now happens when his turn comes up? Not sure.
I'd *expect* him to have access to only his normal attacks and equipment, but still gain the stat / HP boosts of his 'devil trigger'. Could maybe alter the main script to activate counter-attack abilities that can still draw from the limit skills, though.
Vincent does not go into berserk status, though limiting does remove berserk/ frog, more as a sanity check than anything else, though.