I've posted earlier that Sol would have tried to kill Vito even if Sonny had kept his mouth shut, Vito's "no" put Sol's back against the wall--he either would have had to return to Sicily empty-handed, or else try to kill Vito and negotiate with a greatly weakened Corleone Family. Vito's big slip was to send Luca to the Tattaglias with the ridiculous story that Luca was "not too happy with our family"