It is no accident that after Michael made him apologize to Zasa that he stuck around to tell Michael that he knew Zasa was angry because Michael was keeping him from rising up in the commission.
Another good point, dt.
It looked as if Vincent had manipulated Michael: he buttered up Connie ("how's my favorite aunt?"), inveigled a meeting with Michael, tried to scare him with tales of Zasa's treachery, and reassured him that he'd take care of it.
Fool! Did Vincent think he could manipulate Michael Corleone, the Master Manipulator? Instead of falling in with Vincent, Michael, in a completely calculated move, threw him to the wolves--calling him "foolish" for not accepting Michael's offer in the "legitimate world," praising Zasa's "good heart," and demanding that he make "peace" with him. Had Vincent gone along, Zasa would have killed him--with Michael's permission. Michael wouldn't have let that happen because letting Zasa get away with whacking a blood relation would have depositioned him.
So, he put Vincent right up against the wall, and Vincent declared war on Zasa by biting his ear. But Michael won. In effect, he told Vincent, "you can't just buzz in and out of my life whenever it pleases you. Now you really owe me. You need to go with me now to save your life." At the same time, the entire incident warned Michael that he couldn't ignore Zasa--and he needed a willing killer like Vincent to protect him. Vincent would become his new Neri. Michael was brilliant.
