1. It's been my view that there never was any real peace. Vito was merely trying to get Michael back home without Barzini and company taking a shot at him, and said what he needed to say to make that happen. He wanted everyone to know that if Michael didn't make it back safely, he would cast blame on 'some of them'. For the Dons, that's a potential declaration of war. From Michael's point of view, any peace that could've been was broken when Barzini caused the death of Apollonia. I have another theory about that situation which I'll elaborate on in a separate thread.

2. In the novel, Michael and Vito make the master plan, but Michael would be the one to carry it out and 'would not allow Vito to even veto it. If he tried, Michael would go his own way.' In that way, Vito would bear no responsibility for it in his own mind. The other new Dons might have seen things differently and would think Vito as complicit in the act for breaking the peace that he promised. They would've seen Vito's inaction against Michael as breaking his word to their families. By waiting until after Vito's passing, Michael ensured that his father could be held blameless for his son's actions.