Welcome, Don Jimbob.
As I posted before, Michael recognized Vincent as a violent pazzo who had limited use to him going forward. I believe he offered Vincent the Donship as a quid pro quo for giving up his courtship of Mary. I think that if Michael had had his way, Mary would have married someone truly "legitimate." He (Michael) would have been increasingly "legitimate" through his domination of International Immobiliare. Vincent would have been the head of a shrinking "olive oil business"--the ever-diminishing "illegitimate" end of the Corleone enterprise. Vincent would have provided just enough threat and muscle to keep the vultures away from Michael. But Vincent would have had no part in Michael's going-forward life.