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.

I think that had Sonny still had Luca Brasi at his side, and a wartime consigliere like Genco, things would have been much different. I think that he just may have successfully wiped out most of his enemies and may have been able to sustain the family during the war until the Don was healthy enough to get back in the chair.

I think he was depending on Sonny to mount a holding action until he got better and could take command.