TB, I think that Michael was growing increasingly frustrated by the dilemma he had created for himself: trying to go straight, but unwilling to let go of the strings. His words to Vincent about having been offered a job before were those of anyone who ends up telling someone "I tried to help" or "I told you so."
At some point, Michael probably sensed a real threat from Zasa: "Well, he knows you’re stopping him from rising up in the commission, Uncle Mike. I say we make him dead. You give me the order, I’ll take care of it." That sensed threat is why at the Don's meeting he sarcastically describes Zasa. When Vincent attacks Zasa in Michael's office, Vincent tells Michael: "All I want to do is protect you from these guys and your lawyers can’t do that.".

Michael's apprehensions about Zasa are real enough that he knows he's vulnerable: "Maybe you should, uhh, come with me for a few weeks. See what happens."