I think Vito understood the greed that drug profits stimulate (per Sonny's gaffe). So, in addition to signaling Sol that it would be pointless to approach others under him, Vito was also assuring that Clem, Tess, Sonny and Fredo wouldn't be tempted to approach Sol on their own (the novel says Sonny was looking for action on his own that would get him out from under Vito's thumb).

Sonny's display of greed no doubt emboldened Sol to take his shot at Vito. And, he probably told Tatt and maybe the other Dons that Sonny's greed meant that he'd accept a deal, and that there'd be no war. Ironically, Sonny's gaffe in front of all those witnesses guaranteed that he'd never accept anything from Sol except his dead body. If he'd made a deal with Sol, Clem and Tess and maybe Fredo would think he was none too sorry to see his father--and his father's objection to drugs--out of his way. It would have wrecked Sonny's credibility as head of the family.