This question's come up before. I think the general consensus is that Vito wouldn't have ordered Fredo's death. Vito knew that Carlo was complicit in Sonny's assassination, but apparently he couldn't bring himself to order the death of his daughter's husband and the father of his grandchildren. Michael likely would have waited for Vito to die, just as he waited for his mother to die. Quite a few people here think that Sonny would have beaten the s**t out of Fredo, but not killed him.