That thread is an excellent read, Turnbull. I've read it a couple of times and actually read it just before I posted my 'possible scenario' with Sonny acting in place of Luca.

Luca's just not the man for that job. Every toolkit needs a hammer, but a hammer isn't always the most appropriate tool. Vito essentially used his hammer where he needed a screwdriver and nearly paid with his life for it.

If Luca Brasi walks into that bar with anything but murder on his mind, he's dead. I'm not sure that the pretense in that case would've mattered to Sollozzo/Tattaglia. At that point, they need to eliminate Vito. Before they can do that, they have to neutralize Luca. He could've called Tattaglia and told him that the Corleones haven't bought enough Girl Scout cookies from his daughter, and she really wants that prize bike. They would've told him to bring all the Thin Mints he can carry. (Why he felt that he needed to wear body armor, yet go in unarmed is questionable in and of itself.)

I don't think that there is a man within the Corleone ranks that would be a 100% fit for the job of spy in this case. I mentioned Sonny because of his gaffe at the meeting creating a precedent for coming to see Sollozzo. He presents himself as dissatisfied, slighted and ambitious. If he can gain some level of trust, he may gain some information that could help the Corleones down the line. That's a big if though.

It begs the question though, what did Vito think that Sollozzo had 'under his fingernails'? The only thing I can surmise is that Vito may have thought that someone more big time than Tattaglia was pulling the strings behind the scenes and he wanted to see which of the other families were putting in on the deal besides the Tattaglias.

Turnbull's solution with Luca and the drug dealer in the linked thread would've been the most prudent.