Barzini and the other Dons at the meeting know, Vito would know they tried to kill Michael

Bruno Tattaglia cancels out Vito's shooting

There is still Sonny's murder and the attempt on Michael's life which killed his wife
I reckon no one was buying the peace talk and that Vito was forgoing the vengeance Everyone was biding their time

Fabrizio was making sure Michael would be the one to drive that car rigged with the bomb but no doubt couldn't do anything to get Apollonia out without raising suspicions