Barzini probably told him that Vito would never make a deal if he knew that Barzi was Sol's partner, so he said, "Go to Tattaglia. He's a pimp--Vito's not afraid of him. Tell Tatt anything you want, but I'll be your silent partner."
But why would Sollozzo first need to go to Tattaglia instead of going straight to Vito? After all, when Vito asked what the Tattaglias would get out of the deal Sol said: my compliments.
So, if Vito had said yes, his capital and his police and political protection for Sol would be making Barzi stronger, putting him in a better position to move against Vito.
But if Vito had given Sollozzo what he wanted, what would he have to gain by betraying him to Barzini? And Sollozzo could have easily doublecrossed Barzini once he was in business with Vito.
