If Vito didn't know until "this day" (meaning until the end of the Dons' meeting) that it was Barzini all along, Barzini must have given clues to Vito. I think it was the way he tried to fix the agreement on drugs. "A refusal is not the act of a friend..."

Was Barzini too self-confident? Could he have kept his guard if he had let do the other Dons like Zanucchi the job? Or even better: Acknowledge the dilemma and pretend to be undecided.