Is this maybe in the book, where Michael says, "That's not how Pop would play it?" Could be a deleted scene, too - I don't know.

I think Vito might have played a longer game, pushing the non-combatant families to push the Tattaglias to give up Sollozzo. Perhaps even pushing Barzini into action against the course he was secretly pursuing. Tom probably anticipated what he would do by cautioning against anything that would pit the Corleones against all of the other families.