I've always been skeptical about he reasoning advanced for the move against Vito. I think it comes down to Barzini coveting Vito's hegemony over the NY underworld and using Vito's No as a pretext to back Sollozzo's move against Vito. But, the drug business was going to continue with or without Vito and Barzini was going to reap its rewards either way. If Barzini could get rid of Vito in the meantime, so much the better for Barzini.