DB Heroes - Vegeta (Super Saiyan 3 Maoji)

maybe you can have babidi grant vegeta more power in return of vegeta obeying him, but it wouldn't work on our prideful prince, so instead of killing goku, he just knocks him out, and goes to fight buu... after vegeta gets tired of fighting with buu in ssj2 level, he uses the form in hopes of crushing buu (he assumes that buu is weak and that he doesn't need to be any stronger, but he's proven wrong later on), the strain it puts on his tired body halfway in the transformation weakens him and reverts him back into ssj2, where he'll decide to do the ultimate sacrifice.
once he's revived i guess you could have him use it against super buu, but it would be useless. with goku being passed out during vegeta's transformation, he is unaware that vegeta was about to transform, the others are aware and are surprised that goku succeeded, because he wasn't tired when he attempted it as opposed to vegeta. in their fight with super buu vegeta explains that he got the ability against majin buu but was unable to use it due to his body.
that's the best explanation i can give for vegeta to use the form in his controlled state.