As I've said before, it looked like Vincent was manipulating Michael. He crashed his party and buttered up his "favorite aunt" for an audience, with Connie reminding Michael, pointedly, that Vincent was "Sonny's boy." But could Vincent vanquish the all-time champion manipulator? Not a prayer! Vincent succeeded in rousing Michael from his Papal Knight reverie by reminding him that he has a powerful enemy. So Michael put him to the test: he called in Zasa and promptly humiliated Vincent ("that was foolish of you...Mr. Zasa gave you a job out of the goodness of his heart...make your peace with Mr. Joe Zasa..."). In effect, he threw Vincent to the dogs. He was testing Vincent--how much did Vincent want to be with him? He got his answer pronto when Vincent bit Zasa's ear, thus tying his very survival to Uncle Michael.

In the novel, Michael asks Vito why he needed Luca Brasi--"an animal like that?" Vito replies that "in this world there are men who run around shouting 'kill me, kill me.' " He says they can be very useful if you make yourself into the only one whom they fear and remain loyal. At that moment, Vincent became Michael's Luca.

Vincent later aquired a patina of civilization and thoughtfulness, as Olivant pointed out in another thread. All that did was expand the range of Michael's ability to manipulate Vincent, as detailed by dt in the original post here.