Well said, DC! We could give FFC and Puzo the benefit of the doubt--that after Michael sternly shouted his disapproval of Vincent's assassination of Zasa without Michael's ok ("I still command this family...that is not what I wanted...never give that kind of order again...") Vincent might have taken it as tutoring and mended his ways. But I doubt it. I'm with dt here--and I'm saying it's another example of poor direction and poor continuity.
DC, to your point about the movie being "only" three hours long : One of my erudite movie pals, after seeing it with me for the first time, remarked, "There must be 18 hours of film--whole scenes and chapters--on Paramount's cutting room floor."