BTW: It was the wrong advice. If Vito had died and Sonny made the deal, Clem and Tess, whose loyalty he would have needed, would have lost respect for Sonny because his gaffe at the Sol meeting had emboldened Sol to take his shot at Vito. The troops might even have believed that Sonny welcomed his father's demise so he could be on his own and get into drugs. Sonny would have needed to fight Sol even if Vito died, just to redeem himself.
I don't know about that. Vito's refusal was wrong or, at least, shortsighted. While he may have been right in the long term that drugs would destroy them, there was too much $$$ there for anyone to refuse, as Don Zaluchi pointed out at the Don's meeting. Drugs were coming, and Vito's attempt to bar the door against them only cost the Corleones a lot of money and blood before he capitulated.
In fact, Sonny making the deal may have cemented the loyalty of Tessio and Clemenza. If the concern was that Sonny was too much of a hothead, him putting good business over emotion may have calmed the Capos' concerns about his temperament. He could always avenge his father down the road.
