One other point about Michael using Vincent:

In the novel, Michael asks Vito why he needed Luca Brasi--"an animal like that?" Vito replies that there are men in the world with violent tempers "demanding to be killed...they shoult 'kill me, kill me'...a Brasi is a powerful weapon to be used. The trick is that since he does not fear death and indeed looks for it, then the trick is to make yourself the only man in the world he desires not to kill him."

It strikes me that Vincent fits that description broadly. He comes charging into the party, scuffles with security, tells Michael that Zasa is his sworn enemy, says he'll kill the infinitely more powerful Zasa, then insults Zasa and bites his ear--the classic invitation to mortal combat. In effect, he's shouting, "kill me, kill me." Michael cannot have failed to notice that. Perhaps he was thinking of his father's advice about using "a powerful tool."