k, in the show, all the chars could shoot like 40 or 50 shots no problem, no chargin, just bam, done. And vegeta is the king of rezoku. So why not give him a thing where he fires nonstop as you hold down fire (up to a max of 12 ki blasts) and then, depending on how many shots you fired, he has a cooldown (more shots longer cooldown etc).
Anyways, its pretty hard to hit a player with rezoku if hes moving around anyways (well, theres always the noobs who fly straight at you, but they deserve to get owned). And if 12 shots is too much, tone the damage down a bit. In the show, vegeta would go insane with rezoku, itd be kinda nice to reinact that in esf.