it would have been such a big mistake for Vito really not knowing who is his real enemy, it makes me believe that line "it was Barzini all along" is much more a cool quote thrown in there than such an important statement. the Don had to know WAY before the meeting all the details about all that was going on with the families, or he wouldn't call the meeting in the first place. we just try to dissect the film and sometimes we still can not come up with an explanation - "Michael Corleone says hello"...

Vito probably means that the meeting served to show him in front of his face how Tattaglia was really just a subordinate for Barzini, like it was already said, "whining to Barzini", and accepting the role of Barzini as the counterpart of the great Don Corleone. that's probably why Tattaglia just had to swallow the death of his son.