I don't recall Michael telling Sonny to check with Vito, whose condition was too grave for him to give any counsel at that point. Michael did initially say, "Wait, Sonny." (But, after McCluskey pulled Vito's guards from the hospital, and broke Michael's jaw, he changed his tune: "You can't wait.")

I agree that Vito probably would have tried to prevent an all-out war and reined in Sonny. He probably would have tried to appear weak, in order to fool his enemies and prepare for a counterattack at a time and place of his choosing. As for Michael, we saw him counsel the same approach in III, after the Atlantic City machine gun attack.