After the Commission meeting, Vito tells Tom: "Until this day, I didn't know it was Barzini all along." Perhaps he saw the same hints that we saw at the meeting: Barzini sits at the head of the table, leads the discussion, jumps to the concluding statement ("Then it is agreed: The traffic in drugs will be permitted, and Don Corleone will give it protection in the East"). BUT:

How could Vito not have known, way before the meeting, that it was Barzini all along? In explaining to Tom how he came to that revelation, he says, "Tattaglia is a pimp...alone he could never have outfought Santino." Well, Tattaglia was a pimp way before the war began--didn't Vito know that then? And, at the beginning of the Commission meeting, he says, "I want to thank Don Barzini for helping me to arrange this meeting." Since the beef was between Vito and Tattaglia, why would Vito reach out to Barzini, if not to acknowledge that he was the key belligerent? Maybe Vito was slippin'...?