Vito's committment to Johnny about the movie is expressed much more strongly in the novel than in the film. Remember, Vito controlled the unions and could have brought Hollywood to a stop. As the novel says, his relationship with Hoover was all official; it wasn't personal. Given Hoover's denial of the Mafia's existence, it's unlikely he would have been of any use to Woltz. Ultimately, Vito might have had to kill Woltz.