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.
This is an example of the novel better developing this portion of the plot when compared to the film. At the end of the Commission meeting, Vito tests Tom to see Tom's reaction to Vito making the peace. Tom astutely replies that he cannot believe that Vito gave his enemies the victory they appeared to receive. Hagen said "so how can I approve or disapprove?" Hagen felt all the clues Vito provided to an upcoming day of reckoning.
Vito was merely making a tactical retreat in his decision to the Commission. Vito was formulating a complicated plan to exact his revenge for Sonny's death, which involved the safe return of Michael. With Michael returned from exile, Vito could keep his peace promise to the Commission while also using Michael to wipe out the other Family Dons. Vito knew that Michael was underrated by the enemy and that he could use this as a huge strategic advantage. In addition Vito knew that Michael would naturally want to avenge the deaths of Santino & Apollonia.
Also a result of the meeting as explained in the novel: Vito wasn't going anywhere. He gave instructions to Hagen to buy all the adjacent lots near the Long Beach mall. He instructed Hagen to do it slowly so as not to alert the enemy. Vito explained "I now wish to live in a fortress."
