I think that Vito's intention was to join together Sonny's forecfulness with Tom's intellect to form one good successor for himself. If Sonny hadn't been killed, I think that upon Michael's return that Vito would have tried to rehabilitate Michael's reputation, and place him at the head of the legitimate side of the family. Having carried out a hit, Michael may never have become Senator Corleone, but he'd have become a bank president or some such while providing advice whenever the family needed it.