I was watching Godfather I today since it's been a few years. I don't know why I never questioned this, but, how does Vito know that whomever brings Michael the Barzini deal is a traitor?

https://www.youtube.com/watch?v=zjwc55InkxQ

His experience.

ehhh that's a little deus ex machina, don't you think?

Not really IMO. Vito is a mobster, he knows how things work; it also goes back to that "Sicilian Cunning" that is mentioned and which Tom lacks. Also, while it's not touched on in the movie in the novel when he was at war with Maranzano, he won the war due to Maranzano being sold out by one of his own guys. So it can be inferred that he knows from past experience because he pulled that same trick on Maranzano years ago.

I think it starts off a bit earlier, when Vito calls the grand meeting to end the Tattaglia war and allow Michael to come home from Sicily (right after Sonny gets killed).

Tom Hagen: When I meet with Tattaglia's men, should I insist all their drug middlemen have clean records?
Don Corleone: Mention it, don't insist. But Barzini will know that without being told.
Tom Hagen: You mean Tattaglia...
Don Corleone: Tattaglia is a pimp. He never could have outfought Santino. But I didn't know until this day that it was Barzini all along.

It was at that meeting that Vito realized Barzini engineered the hit on Sonny by getting Carlo Rizzi to lure out Sonny into the open. That's why he warned Michael about the Barzini deal -- he'd have to flip another Corleone member to kill Michael.

awesome, thanks. Kind of surprised I didn't put it together when I watched it.

And if you look closely at that scene, Barzini gives himself away. He, not Vito, sits at the head of the table. He, not Vito, takes the lead in the discussions--and he jumps the gun on Vito by declaring: "Then it is settled. The traffic in drugs is allowed, and Don Corleone will give it protection in the East. And there will be the peace." (He's also the only Don smoking a cigarette--the others are smoking cigars.) I think Vito deliberately let him show his hand.

The only thing that surprises me is why it took Vito so long to figure out that it was Barzini all along. "Tattaglia is a pimp; alone he could never have outfought Santino." Well, Tattaglia was a pimp all along--why didn't Vito see that Barzini, younger than the other Dons and more ambitious, would have the most to gain?

I guess he wanted proof of it? Vito strikes me as the type of man who wouldn't kill someone on suspicion alone. Maybe he needed to see Barzini exulting in his victory, like you said, to know for sure.

Could be. But, all the other families lined up against him during the war.

This is not really so implicating, is it? Vito opens his own remarks by thanking Don Barzini for helping Vito to organize the meeting. Vito's a party to the dispute, and so can't chair the meeting. It could actually be that Vito learns more from the silence of Cuneo and Stracchi than from anything Barzini says.

It absolutely is. Especially coming from a man who lacked the cunning to know he was throwing Luca Brasi into a death trap.

When watching the GF again this afternoon, I noticed Vito's words after he consented to protect the drug trade in the East. He said "... I will not be the one to break the peace we've made here today." He could have said, "I will not break the peace ..." By stating the former, was he prophesying that the peace would be broken, but not by him?

Could be -- he was probably playing the long con rather than the short. By not breaking the peace, it was carte blanche for the other families to move in on the Corleone's turf, thinking Vito was weak.

In the end, Vito kept his word. It was Michael that struck back with a vengeance.

Oli I think he used it loosely to show he was truly sincere, but I think at that point he knew Michael was going to take over and he knew (I think) in his mind that Michael would get revenge because they killed Sonny and of course tried to kill Vito. By saying that, he knew they would eat it up as Barzini thought by Vito giving in that gave him more power and it also brought drugs in like they wanted with no war.

Getting back to the question at hand, the answer is easy. If one of Michael's people comes to him telling him that Barzini wants a meeting on his turf for safety reasons, then it is clear the bearer of the news is a traitor.

As we know, negotiations between rivals within the mob have their own set of rules, the main one being that the "negotiators" ... i.e. the family whose name I forget, acts as the hostages while a meeting between adversaries takes place. This is the only way real negotiations take place...thats what they did when Michael was sent to meet Tatt.

For a capo to come to the don and inform him he has made the arrangements is not the way its done. Even Tom, who did not always sniff things out knew something was afoot when he asked Michael at the Vito's funeral about whether he knew "how they're going to come at you?" All Michael had to say the mettion was on Tessio's ground "where I'll be safe," and Tom understood right away by confessing he always thought it would be Clemenza, not Tessio.

Well, Vito had almost died, and he was out of the loop. The last real business he did was with Solozzo, and at the time they thought it was quite an accomplishment to have "discovered" that the Tattaglias were bankrolling him.
If you recall, the last thing Vito did was to have Luca Brasi go sniffiing around the Tattaglias to find out what was under Sol's fingernails. Had Vito not been shot, he would have realized it was Barzini sooner...keep in mind the next time Vito did any business was after Sonny died, and within the time of one meeting he realized it was Barzini all along.