I think Don Vito realized Barzini was pulling the strings by the way he took the lead in the meeting of all the families. Barzini acted as mediator but started working Don Vito to loosen his reighns on some of his power contacts. Don Vito said that Tataglia was a pimp meaning he was small time and really didn't need his political contacts for that. Tataglia was not smart enough IMO to operate a drug ring either.

Telling Michael the person who comes to him with this Barzini meeting is the traitor was obvious to Don Vito. The Corleone family was basically sitting back waiting for Michael to make his move as noted in the scene in the den where he tells Clemenza and Tessio to "be patient". Tessio came to Mike and instead of just saying Barzini wants a meeting, he arranged the location even assuming Mike would go ahead with it, in other words making the decision for him.
Think of it in these terms, if an employee came up to you with a suggestion or perhaps one of your children, on occasion, don't you think there is some other kind of motivation behind it? I believe that is what the Don was alluding to.
Great topic!