Vito told Sol that he wished him well in his business "especially since your interests don't conflict with mine." So, I'm thinking, Vito might have disapproved of the drugs biz in general, but as long as he had no direct connection with it, I don't see why he'd start a war to stop others from supporting Sol.
Big question: would he heed Tom's warning that, with the money others amassed from drugs, they'd buy more police/political protection and come after what the Corleones had?
The thing is there would be no way Sol's business would not interfere with Vito's. They would be selling in the territories run by Tessio and Clemenza, and the Corleones would have experienced the same problems that Don from Detroit had. "For years I paid them (his people) extra..." not to sell drugs, but in the end "they can't resist. Since a chain is only as strong as its weakest link, the Paulies of the Corleone family would get involved with drugs whether Vito liked it or not, and then Sol's interests, selling drugs, would be conflicting with Vito's: not selling.
