Why did Vito ask Luca Brasi to pose as an informant to find out what he could about Sollozzo and the Tattaglia family? Wouldn't he be the worst possible candidate for a mission like this? They'd have to be unbelievably stupid to believe that Luca, the Corleone's most fanatical devotee for decades, would suddenly be so discontented that he would switch sides. Why not send a more "average" soldier or associate instead?

We've discussed this elsewhere, but I can't find any of the links. Anyway, you're right, it was an atypically stupid move on Vito's part, and certainly the biggest blunder of his career. Luca's loyalty went beyond whatever vows he took. It was tribal; practically archaic. Sollozzo and the other Dons would never have believed him to be a traitor.

Even more, what is it that Vito thought Luca could uncover that he didn't already know or, at least, didn't already surmise?

I guess I should have said "be an informant"--"pose as an informant" would be pretty self-defeating!

I know it's been discussed before as well..but I think Luca's mission was Puzo's way of showing that the Don (and the Corleones) was slipping (as Sollozzo helpfully noted). The old ways of doing things wouldn't work and their greatest strength was quickly neutralized by Sollozzo.

Everything that's been said above is true. But to address the question as to why an average soldier was not sent instead, well that would have been to much of a risk for Vito to take.

Vito knew that he could trust Luca with not only carrying out the mission that he was sent on, but more so in that Luca would not turn on him or even tell anyone else in the Corleone family what Vito sent him to do.

Had Vito sent an average soldier on that type of a mission, he not only would have ran the risk of that soldier actually turning on him, but he would have also ran the risk of that average soldier running his mouth off to one of his associates which could have and probably would have meant big trouble for the Corleones.

Luca was Vito's most loyal and trusted friend.

Unfortunately Sol and Tags, or should I say Barzini, knew this.

It may have been a risk that Vito had to take. But as said above, he was slippin.

Here is the link to the earlier discussion:

http://www.gangsterbb.net/threads/ubbthr...true#Post444618

Thanks for the link, TB. I would add to this the fact that Hagen made a comment to Sol to the effect that no matter what else he could do with Sonny and the family there was no way he could stop Luca Brasi. Sol, with his back turned to Tom, in a great bit of acting gets this really smug look on his face and says something like "let me worry about Luca," knowing full well Luca was already sleeping with the fishes. This demonstrates to Sol and probably therefor to Barzini that there was certain secretiveness, therefore a weakness in the Corleone family...how could a real consigliere not be made aware of this? From the look on Sol's face you could see he was already devising his next move.

Here, I would like to defend Vito. The move was not as stupid as it looks from the end of the story. Luca's mission was not to play the betrayal of his Don but to play a guy who is a little uncomfortable with the family and his income. I think Luca played his role well. He doesn't agree on the deal too quickly. It takes him months until he gets in contact with Sol.
I think, sending Luca was smart. But Vito couldn't have thought that Luca got killed as soon as he touched Sol's hand. Killing Luca only made sense because it was embedded in a larger move: Making Paulie switch sides, killing Vito, kidnapping Tom. Four moves at a time. It was a risky thing. And unlike Michael a couple of years later, Sol failed. Vito survived twice, and Sol got killed in the end.
A guy like Solozzo was too bold. Too bold for Vito. Too bold for himself.
Vito's mistake was that he didn't see Barzini behind all these things

Luca's loyalty to Vito was legendary in the Mafia community of the day. There was little to nothing to be gained from Luca's efforts. On the other hand, Vito exposed Luca to elimination, something that it was unlikely Sollozzo could have otherwise achieved.

Vito wasn't really comfortable with Luca in the family at that point, so Luca was expendable.

Why would Vito not be comfortable with Luca "at that point"?

As the novel states, Luca was the only person who could make Vito nervous. So,it was only in that respect that Vito was uncomfortable with Luca.