I think Michael would have done it while Vito lived. The statement quoted from the novel ("you are to have no part in what is to come...") shows he believed Vito would be living when he pulled the trigger.

Also, as I've posted elsewhere, Michael had no intention of turning over the olive oil business to T&C. For one thing, a dual donship never would have worked. For another, Michael needed to neutralize the other NY families, and maintain his own NY muscle, to protect his position in Nevada. Without doing so, he'd be just another "legitimate" casino owner, easy prey for other Mafiosi (not excluding T&C) who did have the muscle to intimidate him.