Is it really likely that Vito would watched from the sideline as his rivals got rich from drugs? No.
Vito: "and good luck to you -- as best as your interests don't conflict with my interests. Thank you."
Exactly. I read that as a warning. Sollozzo probably did, too.
I think anything that increases the power of another Family would conflict with the Corleones' interests.
At minimum, Sollozzo had to consider the possibility that Vito would use his political might against him. That's why I think the hit would likely have proceeded regardless of Sonny's outburst.
It *was* a warning. Sollozzo's interests (drugs) did conflict with Vito's interests (his political connections).
I think that Barzini knew Vito would refuse. After all, Barzini knew how Vito operated as they had begrudgingly worked together for years (with the rest of the Families) so he knew that Vito was an old "Mustache Pete" who didn't want drugs around. So I think he pushed for Sollozzo to talk to Vito, knowing that if refused Sollozzo would not take a "no" lightly (as he was also a man of respect and couldn't let another man dictate things to him), and then Sollozzo could easily be manipulated into making a move against Vito with Barzini and Tattaglia backing him up.
Either way, Barzini wins. If Vito by some miracle says yes, then Barzini profits, Vito loses some connections who would not want to be involved in drugs (and maybe Barzini, since he's doing things discretely, picks them up instead), and Sollozzo can be a scapegoat later if needed.
When Vito says no, Barzini can move against him. If Vito gets killed, he throws Sollozzo and Tattaglia to the wolves after a war and cleans up the pieces afterwards. If Vito lives, well nobody knew Barzini was involved at that point so he can still throw Tattaglia and Sollozzo to the wolves.
Thing is that Barzini (and Sollozzo and everyone else) underestimated Michael
