I always thought Lucca was the wrong guy to go to find out what Sollozzo had underneath his fingernails. Clemenza or Tessio would have been better choices to serve as spies. Still, I don't fully understand what Vito was thinking when he cooked up this scheme. Say, for instance Sollozzo and Tattaglia Jr. bought Luca's story. Then what? Does Luca go to work for them and report back to Vito? At some point does he leave Sollozzo and return to the Corleones? The whole scheme makes no sense.
Assuming Luca infiltrated the operation, at best he would have learned that it was Barzini, not Tattaglia who was running this drug operation. With that information, Vito would know all the families were lined up against him, and there would have been a war. This happened anyway.