He would have had to govern as long as he could, and would have divided the Corleone territory among Tess and Clem.
Without giving Fredo and Carlo anything?
Well if Sonny is dead then everyone still would have known Carlo assisted so even without a Michael to turn things over to Carlo is not going to get anything from Vito and friends except a quick trip to the afterlife. Vito may have still wished his friends to wait on that until after he's gone but he's not a forgiving person.
Fredo was incapable of standing up for himself or family interests in Vegas so even without a Michael to chastise him, I don't see Vito giving Fredo any real authority or power in the business.
Considering the types of people in Vito's world, the best way to protect Fredo after Vito was gone may have been NOT to give Fredo anything anyone wanted.