Other mistake: The close up of the Wall Street Journal mentioning Immobiliare seems to have operating instructions for a Windows program under the headline. The newspapers previous to this appearance also have a headline pasted onto another news topic altogether. The same happens later with the newspapers covering cleansing actions of the new pope. These papers date from March 27, 1980, although the events are supposed to take place in 1979. (00:41:42 - 02:00:00)
Other mistake: Before Joey Zasa is murdered, an altar boy in the parade (on the left side of the screen) can't help but stare at the camera all the time.
Other mistake: In the Mafia World, Vincent would have never been appointed boss of a family without first being initiated as a full member of Cosa Nostra, working his way up the ladder from soldier to Capo, etc. As far as we know, Vincent, from the beginning, was only an associate working for Joey Zasa, then starts tagging alongside Michael, who gives him control of the Corleone Family later, after his failing health.
Other mistake: In the mafia world of Cosa Nostra, anyone that puts their hands on a made member, especially a low-level associate such as Vincent, would be immediately killed. Vincent would not have been allowed to live for viciously assaulting a mafia Capo, as was the rank of Joey Zasa in the film. Not even Michael could have saved him from such a fate had this been true.
Answer: Michael is trying to divest the family of all ties to organized crime. Internazionale Immobiliare is an international real estate concern and owning it would legitimize the family, making them respectable landowners. Michael is attempting to buy out the Vatican's 25% of the shares in it to gain controlling interest, but the other families either want in on the deal or want to steal it out from under him, forcing him back into his old criminal ways.
Captain Defenestrator