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'...?
I always took that to mean that Vito knew that
someone was backing Tattaglia but, until the meeting, he did not know who it was. (BTW, I don't think Vito says "alone" when talking about Tatt outfighting Santino, TB, but the sentiment is certainly there.)
I agree that Barzini seemed to be the most likely suspect, but Vito probably wanted to make sure of it before he took any action. After all, his primary motivation at the meeting was to ensure Michael's safe return.
