Good point about before and after events of the Commission meeting and the peace agreement

As regards Tessio-Barzini plot (a la Luca/Tattaglia) would Vito be "ridiculous" and “incredibly stupid” again?! Surely not!
[borrowing from waynethegame Ref: Michael and Pentangeli's conversation]

and "always smarter," Tessio should have had more faith in his Don and "be a friend to Michael."
[borrowing from mustachepete Ref: Corleones destiny “Characters have to be credited with free will” to mess up! “or there's no story to tell”]

As such, Tessio's betrayal was pivotal in the massacre....All the other Dons were completely relaxed and confident that Michael was on his way to his death and were going about their routine as normal and made fairly easy targets for Michael's men
They were all taken by surprise as the Corleones had intended Perfect!