I don't think Vito suspected anybody explicitly, he knew that only a handful of people would have been able to come up to him directly with the request; but any number of people lower in the organization, for that matter, could have requested the message delivered to him in person, so it could have been anybody, as the incident with Paulie Gatto proved earlier in the movie. By your logic this would place greater emphasis on Al Neri, Rocco Lampone as being the most likely person to betray Michael. Vito and Michael were very logical and I know that they all things being equal nobody would be able to be ruled out in Vito’s or Michael’s minds, but in terms of historic data, less emphasis would have been placed on Clemenza and Tessio due to their track records.

The logic behind Vito's statement is the person who comes to has had prior contact with Barzini, who is still scum. This prior contact is indicative of betrayal, Barzini, is power hungry and a conniver and the uses people to do his bidding. The person who comes to him will have his hand in the pot and have a hand in the betrayal, by doing Barzini’s bidding for a pay off. Nobody within the organization in other words would have enough interest in and of themselves to become peace makers, if they come as peace makers they are wolves in sheep skin.

As for Tom's comment, He thought it would have been Clemenza (out of the 2 capos, he means). To Tom Clemenza seemed more likely to betray the family than Tessio. This comment just allowed the viewer to get more of a sense of the distinction between the two Capos. Clemenza, although not as cunning, was still more loyal. As for Tessio, he was smarter, and this logic is what made him attempt to betray Michael was more of a "business" move (although the term business is a euphemism, used by mobsters to make hard decisions, like murder easier and even justifiable).

Second of all no matter how noble and honorable Vito was, he still played the Mafia game, he wasn’t a good person, although he does nothing we directly disapprove of in the movie, he is a criminal, maybe not a psychotic killer and drug dealer, but still a killer, a thief, and extortionist, etc. The code of the Cosa Nostra is clear, whoever betrays you in the organization he goes. From a personal standpoint if my old friend is going to betray my son, fuck ya, I’m gonna whack him before he whacks my son.