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.
It seems most likely to me that at the time Sollozzo meets with the Corleones, Barzini has already told Sollozzo, "I can't touch this unless the Corleones are involved." Under that scenario, Barzini's worst outcome is that Vito agrees and Barzini makes money. More likely, though, is that old-fashioned Vito says no, leaving the forceful Sollozzo a choice between going home and trying to eliminate Vito as an obstacle - and that's really no choice at all.
