Since no one has shared their thoughts, I'll give mine:

I think Michael would have let Zasa kill Vincent. Let's face it: A guy who had his own brother killed wouldn't have many warm-fuzzies for an illegitimate nephew who he didn't even invite to his party, whom he offered a good job with him in the legitimate world and who turned him down to take a job with Zasa, and who foolishly provoked and took on a much more powereful man. More important: Michael had only hours before been "blessed by the Pope," and had just forked over $100M to Archbishop Gilday for charity. Why would he let Vincent drag him back into the gangster world? As he said to Vincent, "So kill him--what does that have to do with me?" Same would have applied to Zasa: As long as Zasa didn't explicitly say he was going to kill Vincent, it'd be a case of "don't ask/don't tell." If Zasa had killed Vincent, and made his body disappear so as to avoid embarrassing Michael, I think Michael would have let the whole matter pass.

But, Vincent biting Zasa's ear was a step too far in that scenario--Vincent forced the issue. I also think Michael, ever the manipulator and user of other people, did have it in his mind to test Vincent. And Vincent showed that he'd put his life in jeopardy for his uncle. There was a guy Michael could use, especially since Neri looked like he was ready for assisted living.