Heh. Now's when I admit I haven't actually played your mod yet, just been eyeing it off - I haven't played FFVII since I had it on PS1, and I'm on my first playthrough since then now, with just graphical upgrades and minor tweaks. Considering whether to finish that, or wait for New Threat 1.5 and play that. So I can't really speak too much to balance.
Putting it as a Tier 3 isn't that abusable, I don't think - way less than allowing it as an option at Tier 1, simply because of how much more it takes to fill a Tier 3 guage. I don't know much about how far you can get into VII's workings, but could you up how much Vincent's Level 3 gauge needs to fill, without upping anyone else's? Or were they smart, and linked them all to the one variable? It's probably not possible to have Vincent not empty his guage...
Idea! Is it possible to make a new form for Vincent, one that doesn't steal control from the player? If that's possible, would it be at all possible to have said new form 'set' at the start of each battle, as a duplicate of the current Vincent build? So since he's in a 'Beast form', Vincent's Beast forms dont' have Limit bars, right? Or is losing control built in to how Vincent's Limits work, too?
Hm. If that doesn't work, and assuming you can't replace the Chaos item with an item that teaches a Tier 4 [No Limit], without requiring Vincent to know all his limits prior to that, and put it in the player's inventory when they get him out of the coffin... maybe bumping every limit but the existing Tier 1 down a notch, so Chaos is the Tier 3, and putting the [No Limit]-teaching item as the Tier 4 unlock, representing him mastering himself or something? Leaves the player still able to get a little extra speed out of him, but it's a Tier 4 gauge fill, not a Tier 1.
Hm. Again. Setting your Tier resets your gauge to zero, even if you set it to the same tier. Could you hang a hook so that when Vincent's gauge reaches just short of filling on the [No Limit] tier, whether that be 3 or 4, it uses the normal set-tier functionality the game already has, so Vincent never reaches his Limit? And dress it up in some lore about him mastering the Beast inside, but in doing so having to give up his Limit, as the necessary control, hard-won through meditation and stuff, precludes him from building up the [whatever] that allows for Limits?
Sorry. Llike I said, I'm not sure of how much you can do with VII's internals, so none of my ideas may work. I'm... cautiously optimistic? the last one might, though.