The Godfather Part II isn't a prequel. A prequel has to be entirely set before the events of that which it's ... erm ... prequeling (hey, I just made up a new word). Godfather II uses the young Vito story to parallel Michael's story, which is the sequel.
Look at it this way - if ROTJ had included flashbacks to a young Anakin, detailing his fall to the dark side, would we consider that to be a prequel? I don't think so.