Vito did mean "for it to happen before Michael was set up for elimination..." No, he did not have to be alive when it happened. He was complicit in its planning and that breaks his word. It didn't matter what Barzini and Tattaglia did nor did not do - as far as Vito was concerned they were dead men. Had Vito lived beyond '55, Barzini and Tattaglia would have died as a result of his plan regardless of who pulled the triggers.