That may be the understatement of the year, UJ!

From the film, the sequence appears that 1) Sonny is killed; 2) Vito is told that night, says "This war stops now," and orders Tom to arrange a meeting of the heads of the Families; 3) Don Tomassino tells Michael sometime (but not a lot) later that Sonny was killed, and he has to leave for Siracusa "subito";
4) Apollonia is killed when the car that, presumably, was going to take them to Siracusa is blown up by Fabrizzio. I'm guessing that all of this transpired in less than two weeks.

But, it probably took more than two weeks to arrange the Commission meeting, so Barzini's attempt to use Fabrizzio to kill Michael didn't violate an agreement with Vito because there was no agreement at that point. Barzini may even have gotten the idea of killing Michael after he was notified that Vito wanted a meeting in order to further weaken Vito's position at the actual meeting, On the other hand, Barzini could have started arrangements to have Michael killed before Sonny was killed.