I agree with DC's reasons and I want to add another:
Michael was the master manipulator and people-user. He saw Vincent's lust for power as being potentially greater than his lust for Mary (as DC said). But he also saw a way to use him:
If Michael'd had his way, Vincent would whack all his foes, leaving Michael to take over Immobiliari and complete his transition to the "legitimate" world. Then Vincent, who was a pazzo anyway, would essentially run all the "illegitimate" stuff that Michael was no longer interested in, and was jettisoning. Michael was giving away the sleeves of his vest. But Vincent would still be his ally, his pit bull in reserve, to scare away potential encroachers on his legitimate territory.
A brilliant ploy! So what does Michael, the genius do? Told that Sicily's top assassin, "a man who never fails," is gunning for him, Michael brings his beloved daughter and ex-wife with whom he's trying to reconcile, to sit next to him in a public theater. Three sitting ducks, all in a row.