One of the major subthemes of GFIII is how Vincent and Michael use each other. Vincent cleverly slithers his way into Michael's presence by crashing Michael's party and buttering up his aunt ("Who's my favorite aunt?") to get an audience with Michael. But Michael, no dummy he, already knows that Vincent is the new generation of Mafia climber. Vincent confirms for him what he already suspected: Zasa wants Michael out, perhaps dead. Michael wants out, doesn't want dead. So he puts Vincent to the test: "Make your peace with Joe Zasa." Michael can't lose: If Vincent caves, he's a wuss--of no use to Michael, who can use Neri to protect him. But if Vincent's really ambitious, let him show his loyalty--and his willingness to take risks to get ahead.

So, he's really saying to Vincent, "Your move, bastardo." And Vincent bites Zasa's ear. Now Vincent has put the ball back in Michael's court. He's saying, "Is blood thicker than water, Uncle Mike?" So Michael steps in and says, "Maybe you should come with me a while." He's saying, in effect, "Just like my father used Luca Brazi, a certified pazzo,, maybe I can exploit you, Vincent--another certified pazzo."

IMO, the rest of the movie shows Michael recognizing that his "retirement" is full of dangers, but he has dear nephew Vincent willing to take lunatic risks for him to assure his physical safety. If Michael had had his way, he'd have gone "totally legitimate." Vincent would have been the head of an ever-diminishing "olive oil business"--the violent, gangster end of what Vincent (and Michael) regained after Zasa's murder. Mary would have married someone "respectable," and Vincent would have faded into the background.