Yeah, also one of the keys to Vincent is that he really has two role models to pattern himself after - Michael, who he actually saw operate as Don, albeit when he was older and to some extent slowed down by his health problems - and the idealized memory of his father, who he knows only through what he has been told by his mother Lucy, his aunt Connie and Michael.

The GF IV plot lines I've worked on feature the return of Santino Jr., who would have actually known his father, even for only a few years, while he was still alive, and would actually have remembered Vito from his childhood years.

Santino Jr., then, would probably more actual memory himself of the characteristics of Sonny and Vito, than Vincent would, which would make it more interesting if he became involved in the family again.
Maybe Santino would work along with Vincent? Would Vincent be willing to accept that? Or would Santino challenge Vincent for control?
It would force Vincent to become a more developed character.