Yeah, Vito's statement, "Until today I didn't know it was Barzini all along" astonished me. How could he not know it was Barzini all along? "Tattaglia is a pimp, alone he could never have outfought Santino." Well, he was a pimp when Sollozzo first proposed his meeting with Vito and said Tat was backing him. More important: Barzini was the obvious number-two guy in the Five Families, younger and more modern than the other Dons--and the one who had the most power and influence to gain from drug profits. Vito should have sussed that out the moment Tom brought the Sol proposal to him.
I think we're operating with a bit of 20/20 hindsight here.
Why would it have been obvious from the start that it was really Barzini behind Sollozzo? Just because Tattaglia, Cuneo, and Stracchi were not as powerful as Barzini does not mean that they did not have much to gain from entering the drug trade. In fact, a risky move like that might be more appealing to a less powerful Don who had less to lose.
Furthermore, look at it from Vito's perspective. First of all, he felt (correctly) that drugs would ruin the Mafia. He felt it was foolish to get involved with. He may have figured that Barzini - a smarter man - would have felt the same way. Tattaglia - a "pimp" -may be stupid enough to make the mistake of getting into the drug trade. Secondly, he felt, like Don Zaluchi, that drugs were disreputable. A pimp wouldn't have the moral standards to avoid the drug trade. but perhaps Barzini would.
It is clear from Vito's statement that he realized someone was backing Tattaglia once Sonny was assassinated - a move too bold for Tattaglia. It wasn't immediately obvious who, so Vito called the meeting to smoke out his true enemy. It worked, and the Corleones won the war.
