My thought on this has always been if Vito had said yes, then Barzini would have benefited the most and would have just postponed his takeover plans, using his backing to get more political power from behind the scenes, grow his empire, and then finally overwhelm the Corleones. Since Vito would have had some of his political contacts drop him, Barzini would have been the likely person to pick them up and then use them against Corleone.