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?
