Vito has to muck around in numbers, bookmaking, extortion and other slimy activities. He's outside of the upperworld. But the money and power he builds up doing this is transferred to Michael who uses it to "purchase" legal casinos and hotels.
Michael straddles the upperworld and underworld but builds even more wealth than his father did. Then the next generation that comes after Michael is completely removed from any sort of criminal activity but has enough resources to own international banks, huge shipping interests, sports franchises, and so on.
A bit of a parallel:
"I must study politics and war, that our sons may have liberty to study mathematics and philosophy. Our sons ought to study mathematics and philosophy, geography, natural history and naval architecture, navigation, commerce and agriculture in order to give their children a right to study painting, poetry, music, architecture, statuary, tapestry and porcelain."
--John Adams
