I missed what Pete pointed out above: the novel states that Vito frequently held a war council and exercised a veto over proposed actions. Thus, Vito must have been aware of what Sonny was doing. As the novel further states, Sonny's war of attrition seemed to be working. So, I can only figure Vito must have approved Sonny's war actions. However, as TB points out, all of that seems to contradict what we know of Vito: he only resorted to violence when he had to. Well, TB may be right that Vito was just biding his time until he was well enough to negotiate and that negotiation would be from a position of strength based on Sonny's war of attrition.

However, if all of that is true, then why did Vito not foresee the Five Families plan to murder Sonny? We tend to blame Tom, but if Vito was as involved in the war as we speculate, then Vito should have seen it coming.