As several of you have pointed out, Vito's capitulation was solely to insure Mike's safe return. Once that was accomplished, really and truly, all bets were off. That's why he put Mike in charge because that way he could technically claim that he had kept his side of the deal. In the meantime, Vito probably accepted fees for his help with the drug trade because money wasn't what he was interested in.

In the novel it says that after the dons meeting Tom was sure that "in the subtle and complex mind of the Godfather a far ranging plan of action was being initiated that made the day's happenings no more than a tactical retreat." Once Vito was dead and Mike was in charge, he probably gave in to reality and tolerated the drug trade while getting a percentage.