I think that Vincent should not go very far; I think that he must be murdered or arrested before Michael dies to emphasize more the misfortune of Michael's destiny. So he would have seen the destruction of " his two families ". When Mike dies (1997) in Godfather III in Sicily I imagine him like a unfortunate man, which has lost his family( Vincent is dead or in prision for example) and he could not have avoided the fall of the Cosa Nostra. Besides, the history would be more realistic because the Corleone family only has had 2 bosses in 60 years, it isn´t very real.