You could possibly argue that IF the murders had taken place while Vito was alive, AND if Michael hadn't been assassinated (or set up for assassination) first.

Two big 'ifs' that never came to be.

And I'm not so sure that it was 'revenge' he was planning so much as preservation of his empire and the life of his son.
I believe it was Michael who was more interested in revenge.

But regardless of what he planned with Michael and regardless of how brilliant that plan was...Vito was not the one to break the peace made on the day of the meeting with the other Families.