I might be dumb, and I probaly still am even if I'm right at this point, but I'm very sure that vegeta can do his final flash always, going ssj (ssj2/3 etc) doesn't give any new attacks. The reason why vegeta never did it when he wasn't ssj was; in the early stages he didn't knew FF yet, after that he only used ff when he was in serious trouble, so then he's ssj too.
ontopic: regeneration would make it realistic, like 1 hp at the time, and only when ur not charging flying (I know in the series they can, but u gotta make it reasonable) or on powerup mode, so when u stand still.
My opinion
grtz Naz