When Vito "makes peace" with Tattaglia during the meeting of the Dons, he took full advantage of their perception of him as weak, without the muscle to go on with the war, and wlling to compromise.

Where Vito was brillliant was his way of taking advantage of this perception. He swears he will never take vengence, and he swears he will go along with the compromise about selling drugs. But he does so only on the condition that Michael is allowed to safely return to the U.S.

Everyone at the table knwe Michael killed Sol and McL. and they also knew that Vito probably had the connections in the legal system to expnerate him of these "false charges."

With this condition Vito makes it very clear that should anything happen to Michael including a feked suicide or a bolt of lightning, he owuld hold people accountable for it. The truth is he did not have the muscle to do this. He knew it, and Barzini had to know it as well.

This was an extraordinarily clever move by Vito. Michael was the only card he had left to play, and he knew that everyone in that room underestimated Michael. He played on this perception and basically got a consensus that Michael would have safe passage home.

As for his threat to hold people accountable, he understood everyone would see it as a matter of pride and old fashioned honor, and that nothing would happen to Michael.

The impression he left was "let the old man have his young sone come home. He has nothing left except a spoiled daughter and an idiot son, and a weak family.
If he wants to feel good about hanging a threat over our heads, so be it, ut lets allow the Ivy League kid to come home."

Vito out foxed them all by getting this favor, and did so from a position of extreme weakness. Noe THAT is cunning.