TB, I've always figured that only several months at most elapsed between Vito's return to the Mall and the Don' meeting. Could Vito have been advising Sonny and Tom? Could be. However, I doubt it. Why? Because Vito would never have countenanced Sonny's continued attacks against the other families and I doubt that Sonny would have disobeyed Vito.

As many of us experience when we are faced with a crisis, Vito reached deep down inside himself to collect all of his strength to do what was necessary to save his family and Michael. However, that effort drained him.