I think Vito figured it out pretty fast at the meeting with the other families; the way Barzini seemed to lord his newfound power over the other Families, believing that Sonny's war had heavily wounded the Corleones, and left Vito with little choice but to capitulate.
Vito should have figured it out a lot earlier. He tells Tom, after the meeting, "Tattaglia is a pimp; alone he could never have outfought Santino." Well, Tattaglia was always a pimp who didn't have the balls or the resources to fight alone. Why couldn't he have surmised that Barzini was behind Tattaglia?
