What i think out weighs everything else on this matter is Vito had no need for the drug business, people who get into drugs need money because most the time they are younger and havent established many other rackets if any at all. Who did Solozzo come to for legal protection and money? Vito Corleone. Vito had the unions, the gambling, and the connections, the only person with nothing to gain from this proposition is who? Vito. By agreeing to protect and finance the drug business he would basically be helping the competition develop more connections, protection, and money. Once they have done that who do they no longer have a need for? Vito. Saying yes or no would pretty much end in the same way, with attempts on Vito's life. So going along with his moral values probably seemed the best way to go considering there would be more work and the same ending to saying yes to the deal.