We all know Michael was angry with Connie and Vincent in Part 3, after they had Joey Zsa Zsa killed. Well, what do you think Michael would have done with Zsa Zsa, had he recovered from his stroke, and Zsa Zsa was still alive?

Not much.
Michael was getting old. He was feeling guilty and trying to redeem himself for his kids, for Kay, and for himself.
The fact is, it wasn't all about the business anymore, it was about him and his family and recoilling it back together. It had nothing to do with a small time boss like Zasa.

Michael, suspected Altobello from after the hit in the comission, he was murmuring his name and shouting how he was betrayed as he was having his diabetic attack. As Michael said beofre the attaack: Zasa was just muscle, a small time enforcer. He didnt' have the brains to think this up himself, the wit to do it with a helicopter or the ambition to have done it in the first place.

Part of Michael's anger with Vincent was he did the enemy a favour by killing Zasa. Zasa, having been manipulated into hitting the commision was now useless to whoever was using him and Vincent killing him acomplished nothing. This point elludes Vincent however and he kills him. It was Michael's hope to get Zasa to talk and find out who contracted him to do the hit.

I agree. Michael never would have given Zasa a pass after that attack, but he probably would have seen him as a means to an end: try to find out who the real traitor was. Just before his diabetic attack, he was telling Neri, Connie and Vincent: "Get word to Zasa that I respect what he's done, that the old order has to pass" (or something to that effect). No doubt it was an effort to set him up to find our more, then kill Zasa.
There's a parallel in GFII: Michael knew with near-certainty that Roth was behind the Tahoe attack, but he went along with Roth--even traveling to Havana, where he was in mortal danger--because, as he told Frankie, he wanted to find out who the traitor was in his family.