We can really ask the question of why didn't Vito order Sonny/Tom to give in and make the same plans with them as he later did with Michael. The story wouldn't have been as interesting of course.
Vito was holding out for his position--no drugs--until events forced his hand. I think he was depending on Sonny to mount a holding action until he got better and could take command. But when Sonny was assassinated, he had to act in a conciliatory fashion--he had no choice. And he had to give in to bring Michael back safely from Sicily, to assume command of the family. Those needs made Vito change course.
