Why did Vito assume that there is a traitor?
When Vito was talking to Michael just before he died. He emphasized to Michael that the person who approaches him with a meeting is "the" traitor.
At this point, there were 2 traitors that we are sure about, Paulie and Carlo Rizzi. I don't think Tessio was at this point a traitor as he was clearly with the family and assisting them with assassinating Sollozzo and seemed to still be friends with Vito and trying to negotiate with both Michael and Vito to get his own crime family.
So first, why did Vito assume that there is an existing traitor? And how did he know that this traitor will approach Michael and arrange a meeting? Or is he just so experienced that he knew how things work after other crime bosses anticipate a weak link?
Add to that, Tom told Michael "I always thought it was Clemenza", so it seems that there was already someone betraying the Corleones other than Paulie and Carlo.
I just watched the movie, maybe the novel has something that the movie isn't explaining well.