Vito's "no" put Sol (and presumably Tatt) in an inimical position: either crawl back into your hole and forget about drugs, or do something else. The something else could have been to assassinate Vito. But, I'm guessing, the other Dons would have been reluctant to go to war against the most powerful family if Sonny hadn't made his gaffe. But Barzini probably would have stepped into the "mediator" role he played later, and demanded a Commission meeting like the one we saw to reason with Vito.

The gaffe emboldened Sol to tell Tatt and Barzini that there'd be no war because Sonny was hot for his deal. Here Sol made a fundamental mistake: while he correctly anticipated Sonny's temper (and brilliantly warned him not to lose it--playing on Sonny's need to keep his head clear as acting Don), he hadn't counted on Vito's survival. What's more, he didn't anticipate that, even if Vito had died, Sonny would be loathe to go for his deal because Tessio, Clemenza and others under him would suspect that he secretly welcomed Vito's demise, and wouldn't support him.