I just saw this thread now:

Sonny could never make the deal with Sol if Vito died, for strategic reasons. First, the capos heard him shoot his mouth off during the meeting with Sol, and, if he made the deal, would think he saw Vito's death as an opportunity to get into the drugs business, which is what he wanted in the first place (the novel said he wanted the drugs deal as a way to get out from under the Don's thumb). Word would get down to the troops. Loyalty to Sonny would erode. Second, Sonny didn't have the hold on the judges and politicos that Vito did. If they'd be reluctant to support Vito on drugs, they'd never support Sonny. Third, Sonny was an excellent "street general" and a formidable fighting force. He had a chance to outfight the other families. He'd lose face forever if he made the deal with Sol.