New poster here (sad I didn't discover these forums years ago), but it strikes me that the Corleones must have had a long-term animosity towards drugs. After Vito gives his consent to allow his resources to be used to aid the drug trade, Michael comes back on the scene, and he and Vito began plotting to take the family legitimate; abandoning the drugs, prostitution, gambling and other rackets the Corleone Family had been involved in.

Vito's agreement at the meeting of the Five Families to make his legitimate connections available to Tattaglia's drug trade was purely self-serving, to allow Michael to return to the States safely. He openly admits that. But I wonder if Vito's plan even at that stage, while Michael was still hiding in Sicily, was to begin intentionally weakening the Corleone Family by capitulating to Tattaglia. Certainly in the other Families' eyes, Vito must have looked desperate; weakened in body and in resolve. Thus his vow that he would not break the peace, a sign of his submission to Tattaglia (and to Barzini, whose manipulations have assured, that at least in the other Families' eyes, he was now head of the most powerful Family).

Finally there is the fact that Coppola and Puzo had at least thought of the a fourth film where Vincent Corleone fully takes the Family into the drug trade, and thus ultimately is responsible for its downfall, suggesting a large arc that begins with Vito turning down the lucrative offer because he feared that it would weaken and even destroy the Family, Michael spending twenty years or more to legitimize the Family to pull it away from all criminal activity to ensure its survival, and finally Vincent ignoring his grandfather's warnings, plunging the Corleone Family head long into the drug trade, and realizing Vito's prediction of where involvement in the drug trade would end the Family.