I heard that the new book implies that Vincent could be Fredo's child. I think I read it on Vincent's profile in Wikipedia.
Yep, here it is...
Within the pages of The Godfather Returns, and followed up on in The Godfather's Revenge, also by Winegardner, it is stated that Fredo had a child before he died, and the child was given up for adoption by the mother. Also, light is shed upon the fact that Vincent Mancini-Corleone is not the bastard son of Fredo's older brother, Sonny, but in fact, that of Fredo.
Example #2,876 why these books are abominations on the Godfather name. Winegardner made Fredo gay, made Kay not have an abortion, made Mike and Kay horny teenagers, Mike a pilot, and made Fredo Vincent's father. Amazing.
He'd be great writing Days of our Lives though.