Zasa came to Michael’s party seeking Michael’s permission to whack Vincent. Michael summoned Vincent, and in Zasa’s presence humiliated him, praised Zasa for giving him a job, concluded by telling Zasa “I have no conflicts, no debts” with him--and ordered Vincent to “make your peace with Mr. Joe Zasa.” Seemed like a one-way ticket to the graveyard for Vincent. Vincent’s attack on Zasa’s ear seemed to have saved him in Michael’s estimation.

Do you think Michael intended to throw Vincent to the wolves—and would have if he hadn’t bitten Zasa’s ear? Or, was Michael putting Vincent to some kind of test—and if so, why? Any other thoughts?

Well, it certainly seems that he had cut Vincent off. Without the very abrupt reversal the audience would start waiting out the death of another family member.

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."

He doesn't seem to be very impressed with Vincent at all to begin with. He chastised him for his clothing at the reception. Connie seems to be the only one in his corner.

Oli, the key word in your insightful post is "frustration." I think Michael was reveling in his Big Day: His Papal Knighthood finally "legitimized" him (as in Abbandando telling the news media, "The Pope has this day blessed Michael Corleone"). And then along comes Zasa, an urgent reminder of Michael's criminal side, asking permission to whack Vincent. And then along comes Vincent, telling Michael he should worry about Zasa because Michael is preventing him from rising in the Commission. So, I think Michael's initial reaction was to try to dismiss both Zasa and Vincent and get back to his "legitimacy," as in, "What does this have to do with me?" And, "So kill him." At that point, I think he would have been happy if they both went away and fought each other.

But, as you say, Vincent rang an alarm bell in Michael's mind. And, since Vincent showed his determination to act by biting Zasa, ear, the Old Manipulator figured that he might as well carry Vincent for awhile--use Vincent as a cat's paw for Zasa's evil intentions.