Michael’s successor as the Corleone family’s don would have felt more natural if The Godfather hadn’t cut a character from Mario Puzo’s book. At its heart, The Godfather is the story of Michael’s evolution from mild-mannered war hero to feared boss of a mafia empire. The Godfather Part III then goes further and chronicles the handover of power from Michael to his nephew, Vincent Mancini. The Godfather Part III infamously lacks the gilded reputation of Francis Ford Coppola’s previous two entries, and introducing Vincent as a secondary protagonist is among the reasons why.

https://screenrant.com/the-godfather-michael-corleone-vincent-mancini-lucy-book-character/