I think that a lot of detail got lost in the movie because the whole course of the war was messed up in the novel. With respect to this question, Vito insisted on the war council being done in his presence so that he could exercise his veto (HA!). Sonny then embarked on his Reign of Terror, and Tom didn't appeal to Vito because the strategy seemed to be working. Obviously, if Vito was included on any level he should have known that his people were shooting every target they could find, and both Sonny and Tom were actively concealing that information from him.

So, in the movie I think that Coppola's answer was to chuck all of that: Vito was brought up to date when he got home, at most was lightly advised of any developments, and rallied his strength only when he heard strange goings-on in the night.