TB great overall analysis of the various points of view, but on this point I would say that Vito DID understand the danger of his "no" to Sollozzo, and it is why he dispatched Luca to find out what Solozzo had underneath his fingernails. I think the mistake Vito made was using Luca as his spy. First of all Luca's loyalty to the Don was legendary, and I don't think Sollozzo or Tattaglia Jr. bleieved the deception for one minute. After all Luca was not exactly the smoothest character around. If anything, by sending Luca to pretend he would betray Vito showed Sollozzo, Tattaglia and Barzini how seriouisly Vito was taking his "no." It also told them that Vito would not simply allow the drug business to flourish as long as Sollozzo's interests did not conflict with his. in other words the real message Luca inadvertantly sent was that Vito had to be taken out.

Thereafter two ironical unintended consequences drive the plot forward. The first was the assassination attempt itself. Vito did not see it coming. The second was the fact that the attempt failed. Sollozzo defintely did not see that coming.