We know from the Novel that revenge was conceived of long before the Barzini offensive. Consider these words directly after the meeting:

Vito To Hagen: “Do you disapprove of any of my deeds today?”

Hagen to Don Vito: “No, but I don’t find it consistent, nor true to your nature. You say you don’t want to find out how Santino was killed or want vengeance for it. I don’t believe that. You gave your word for peace and so you’ll keep the peace but I can’t believe you will give your enemies the victory they seem to have won today. You’ve constructed a magnificent riddle that I can’t solve, so how can I approve or disapprove.”

Vito to Hagen: “Well you know me better than anyone else. Even though you’re not Sicilian, I made you one. Everything you say is true, but there’s a solution and you’ll comprehend it before it spins out to the end.”

Vito’s words clearly indicate a master plan has already been hatched and vengeance planned.

The novel also provides evidence of the Don’s uneasiness about the peace he’s made:

Vito to Michael: I would not have made that peace but that I knew you would never come home alive otherwise. I’m surprised, though, that Barizini still made a last try at you. Maybe it was arranged before the peace talk and he couldn’t stop it. Are you sure they were not after Don Tommasino?

The Don’s uneasiness about “that peace” (not “the peace”, “that peace” – presumably to reference the his swearing on souls ) coupled with his interest in the details of the Sicily hit tell us that Vito was seeking to identify Sicily as the provocation he could hang his hat. He even wanted to double check that it was not meant for Tommasino! Michael seemingly succeeded in pacifying the Don that it was meant for him, and the required provocation.

Don to Michael on Michael’s taking “all responsibility”: “So be it. Maybe that’s why I retired, maybe that’s why I’ve turned everything over to you. I’ve done my share in life. I haven’t got the heart anymore. And there are some duties the best of men can’t assume. That’s it then”.

I think this was Vito playing out his role in the end-game-- to step aside once it was all ready to go. The trumped up weakening of his will was the final attempt to unconnect himself from the massacre. This was the final act of the “Solution” that Vito referenced to Hagen “that has finally spun out to the end.” It was all planned.