Yes, Vito certainly would have proceeded differently if he'd perceived from the beginning that Barzini was behind the drugs deal. The novel makes clear that Barzini intended to supplant Vito, and Vito would have known it. So, if Barzini had vouched for Sollozzo prior to their meeting, Vito would have taken the matter far more gravely--perhaps even made a preventive strike against both.
I've speculated on this board that Sollozzo would have approached Barzini first. He'd have seen that Barzini, as the would-be heir-apparent, would have more to gain from the drugs business than anyone else. But Barzini would have told him, "Look, Vito Corleone is old-fashioned about drugs. If he sees that I'm behind you, he'll not only turn you down but will feel threatened and may come after both of us. Instead, go to Tattaglia. Tattaglia's a pimp, and Vito won't see him as a threat to him. I'll be behind you and Tatt--silently."