Maybe we should interpret Vito's statement a little more liberally. Maybe he meant that he was sure until today. As TB posted, he probably had his suspicions about Tattaglia's inability to pull off such a coup.
Vito's words were, "I did not KNOW until this day it was Barzini all along," so I guess that could mean he suspected it. It could be telling that Vito contacted Barzini to arrange the meeting with Tattaglia. That would be a typical "keep your enemies closer" move.
And BTW, what was it that Stracci and Cuneo did wrong?
