I've always wondered why Vito hadn't figured out until that day that it was Barzini all along. Tattaglia, as he told Tom, was a mere pimp. Barzini was the obvious Number Two Don in NYC, waiting to supplant Vito. Drugs would give him enough capital to improve his position. Vito, with his Sicilian cunning, should have been able to figure out that Tat and Sol, by themselves, did not have the wherewithal to make the drugs deal a big-time proposition. Someone else had to have been backing them...someone who'd hide his interest so he'd be out of the line of fire in case something went wrong, and who'd pick up the pieces in case of a mob war or a police crackdown...someone like Barzini.