Though it never says it explicitly in the movie: Vito wanted Michael home so he could turn over the reins of the family to him, I believe it was MIchael's initiative, not Vito's, to settle all family business before the move to Nevada. In a deleted part of the garden conference between Michael and Vito, MIchael says, "What about Sonny? What about Sicily?" Vito admits "It was a sign of weakness" that he didn't attempt vengeance. I infer that it meant he was leaving that to Michael. In the novel, Michael warns Vito that if he attempts to dissuade him from going ahead with the Great Massacre, Michael would go his own way.