As Q said, early on in GF we learn that Vito's main businesses are gambling and unions. He augmented his income later by agreeing to provide police/political protection for the drugs trade for a fee. We also saw that Michael was muscling Moe Green out of his hotel, so his involvement in the Nevada gaming industry began in GF. The novel alludes to Vito (and perhaps Michael) wanting to get into the movie business, although I think it was still union business rather than making films.

In II, we know that Michael owns or controls three hotel/casinos and muscles Klingman out of his interest in a fourth. Fredo runs a legal brothel. All legit. But: Michael is still the Don of the "olive oil business" in NYC, with Pentangeli as his underboss. Presumably, that's still gambling and unions. Plus, Michael is backing the Rosato brothers who, Pentangeli claims, are into "women, broads...and a babania, junk." Was Michael getting a piece of that? Also, was he still providing police/political protection to the drugs business? True, Vito was forced into it as a condition of getting Michael back home safely from Sicily. But I can't see Michael walking away from a source of profit.