What was farfetched was Vito sending Luca to the Tattaglias with the story that he was "not too happy" with the Corleones. Every Mob guy in NY knew Luca was Vito's man to the death. And everyone knew that Vito could not possibly let Luca, "my most valued friend" (movie), a "man who could do a job of murder all by himself" (novel), ply his unique trade for another family. That's where the Don was slippin'.
Yeah, but only because we know the result. It could have been a very smart move. A trap for Barzini who considered himself to be smarter then Vito. And if every-one thought, Vito was getting old, then it could have been a plausible move for Luca. Look, he never said he was unhappy with the Corleones, he never worked openly against the Don. He pretended to behave the same way Paulie behaved. Looking for extra money.
