I see Frank as a protected son, isolated from the Corleones by his overbearing mother. Maybe he runs a business on his own, but something useful that would give him the cunning and ability to succeed in the organized crime world. Another family member would be a cousin or someone from Sicily...a loose canon type.

Vincent, meanwhile, is stuck between legitimacy and the drug underworld that always seduced his father. In that way he is much like Michael was in part III.