I think the consensus is that Vito wanted Michael to work for him in a capacity outside of the olive oil business. When he enlisted Tom told him that he and Vito had discussed "plans" for Michael. Vito wanted Michael to finish his education, and perhaps get a law degree or a business degree that would land him in a position where he could cement the family's political ties and union connections, but in a legal, or at least quasi legal way. He saw Michael as someone who could eventually move the family money into the "legitimate world" and get them out of the business they were in. Michael hints this when he tells Kay "My father's way of doing things is over. Even he knows that." I think Vito saw it playing out with the deaths or retirements of Clemenza and Tessio, and the dimunition of the street business. Sonny might have ended u as the Don in Vegas moving out Moe Greene, and establishing something out there. But, as Vito said, "There wasn't enough time."