I'm in with olivant; I see no reason that Sollozzo whacks Vito if he has nothing to gain from it. Barzini could certainly offer up the financing and help Sollozzo fly mostly under radar with the drug deals. Perhaps after they build up the wealth, they can buy their own political influences.

A big fulcrum in any potential plan to whack Vito is Luca Brasi, as we well know and which has been discussed at length. If Sonny doesn't speak out of turn, would Vito have sent Luca to try to dig for dirt on Sollozzo? I say no. Vito has no interest in the drugs trade, and without Sonny playing into it, that's the end of it for him.

In the original scenario, Sollozzo knew that he had to whack Luca before he could safely go after Vito. He's a new guy on the block, so it's likely that Barzini or Tattaglia advised him on that course of action. Neither Don would have cause to advise Sollozzo of that fact if they wouldn't sanction the hit though.

In the fait accompli scenario, Sollozzo would go 'lone wolf' and call the hit on Vito himself. If he succeeds, Tattaglia is definitely caught up in the whirlwind, Barzini too if the trail leads back to him. In addition to the potential for war between the three families, they've got Luca Brasi to deal with as well. If Vito's dead, "not even Sonny will be able to call off Luca Brasi." Offering Sollozzo up as a sacrifice at that point might be too little, too late.

Therefore, I'm of the mind that Barzini would have Sollozzo pre emptively whacked if he got wind of Sollozzo wanting to go lone wolf in the alternate scenario.