There wasn't enough time, for Michael, wasn't enough time....to even enrol, to "Finish school" because of this Sollozzo business

Vito's Grand plans
Originally Posted by Turnbull
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
Is it fleshed out in the novel that Vito had no stomach for vengeance? that “it was Michael's initiative” deliberately playing weak, to take Barzini and everyone, by surprise, with their brilliant Great Massacre [with “Vito's guidance and counsel”]

I find this perplexing that Vito would let Sonny's murder [machine gunned like a Swiss cheese] and Michael's attempted murder, resulting in his new wife, Apollonia's death [car bombed] go unanswered by Carlo and Barzini respectively