I agree with most everything said here. Vito really WAS slipping by thinking that the other families wouldn't think something's fishy when Luca Brasi starts to say that he's not happy with the Corleones. Clemenza or Tessio I could see (although it'd be a hard sell since they're such close friends, but hey Tessio did it at the end) if they mentioned something about Tom becoming Consiglieri ("Yeah.. he made the Irish kid consiglierie instead of me, who's been his friend for twenty years"). But Luca? No way!

I'd like to also add, though, that in the novel Luca debates bringing a gun to the meeting in case he gets a chance to whack Sollozzo, but then decides against it and figures he'll just get information, and relay it back to Vito. Luca is excused from this because he wants to follow orders and isn't supposed to be that smart, but he should have known, and Vito should have known, that everyone else is going to wonder why Luca would go against Don Corleone's wishes (I believe the novel also has Luca saying he wants to get a piece of the drug action himself to make more money).