Oh we disagree breh
Separate the two plots and look at them individually, neither can really stand on its own. Now ask what does Vito's entrance into organized crime tell me about Michael's continued ascension into organized crime? We see Vito doing it for family, but we already know Michael does it for himself. If anything, the Deniro as Vito footage would fit better interspersed into part 1. Then we can see the contrast in the two men. Michael's ruthlessness vs. Vito's pragmatism; Michael's ego vs. Vito's legitimate desire to provide. By 2 we already know everything we need to about Michael. Seeing how his father got started doesn't add anything to his story and seeing him continue to do Michael-like things doesn't add anything to his father's story.