I agree in part, but remember Michael made those comments about needing lawyers when Zaza was in the room, so if he had even given bringing Vincent in a thought, he would have kept it to himself because at that point it was more important for Zaza to think that Michael was on his side in the dispute. But notice Mike's Sicilian Cunning in dealing with this. When Vincent says that Zasa is going around saying "Fuck Michael Corleone," and Zaza denies it, then Michael goes to Zasa and gets him to agree that anyone who went around saying things like that would be a "dog." Thus he sets the trap for the lying Zasa.

It is AFTER Zasa leaves the room that Michael asks Vincent why he is so sure Zasa is out to get him and Vincent says "Because he knows you are trying to stop him from rising up in the commission." At that point Michael realizes that Vincent is not just some stupid thug, but someone who knows what is going on, and someone he could teach.