I come out about where Lou came out: Whether or not Sonny kept his mouth shut, Vito's "no" put Sol's back against the wall. He couldn't operate without Vito's police and politicians. Without them, Sol either had to go back to Sicily with his tail between his legs, or kill Vito. With Vito dead, half of his political support would be lost, as Tom noted to Sonny after the shooting. That would have put them into play for the other Dons--and Sol--to "buy." Then Sol could negotiate with Sonny for the police/politicos remaining loyal to the Corleones, and at a reduced rate because the Corleones would be gravely weakened with Vito dead.

The one big difference: Sonny's show of greed at the meeting emboldened Sol to take his shot at Vito--and no doubt to get the backing of Tattaglia and probably the other Dons. They would be wary of starting a war. But Sol told them: "There isn't going to be a war because Sonny is hot for a deal, and so is Hagen. He'll convince Sonny to cool his temper and accept a deal." If Sonny had kept quiet, Sol would have had to try harder to convince the Dons to back him in killing Vito. But, there, too, greed carries the day: their own greed for drug profits would have convinced them to back Sol. And, even if the other Dons were reluctant, Sol would have still tried to kill Vito and present the other Dons with a fait accompli, and the promise of drug profits.