Vincent attempts to ingratiate himself with his uncle by protecting him from rival Mafia families, who are in league with Zasa and an unknown traitor within Michael's circle.

Vincent saves Michael from an assassination attempt orchestrated by Zasa at a Mafia summit in Atlantic City.

Believing Zasa will make another attempt on Michael's life, Vincent murdered him (with approval from Connie and assassin Al Neri).

Michael is angry that Vincent used violence to deal with Zasa and did so without his permission, although he used the same logic to protect his father 34 years prior by killing Virgil Sollozzo.

Vincent learns that Licio Lucchesi, a powerful Italian politician and criminal underworld figure, was the mastermind of the assassination plot against Michael and employed Altobello, Zasa,(a corrupt Vatican official) Frederick Keinszig, and Archbishop Gilday to undermine Michael's criminal empire.

In The Godfather Part III DVD commentary, Coppola explains that both he and Puzo had envisioned a story depicting Vincent's reign as head of the Corleone family during the 1980s, still haunted by Mary's murder.

The film would have portrayed the early days of Tom Hagen, Luca Brasi, and Johnny Fontane, and Vito's first meeting with Hyman Roth.

According to Coppola, Puzo had composed a rough draft alternating between Vincent's reign as boss and the "Happy Years" of 1926–1939; passages from the original novel not previously incorporated into the movie storylines.