I wondered about that, too, because Vito swore that he would not be the first to break the peace that was made at the Commission meeting. But Michael wasn't bound by that pledge. And, when he and Vito were in the garden together, Vito warned Michael: "Whoever comes to you with this Barzini meeting--he's the traitor." That tells me that Vito was in on the planning for the Great Massacre of 1955. And, since the Barzini meeting seemed imminent and Vito wasn't planning on dying, I guess the massacre would have gone ahead if he had lived.