Vito needed Michael to succeed him because Sonny had been killed, Fredo was out of it, and he was physically and mentally spent. Michael's apprenticeship began when he returned from Sicily (probably ca. 1947) and by 1954, when he visited Moe Green (car that takes him to the hotel is a '54 Imperial limo), the Don had "semiretired" and Michael was head of the family. But I think he downplayed that transition because, after Vito died in 1955, Michael wanted to appear weak--the new guy in charge--to trap Barzini. It was all carefully arranged.

By contrast, Michael was distainful of Vincent at the beginning of III, then rather cynically (IMO) began to use him as a kind of cat's paw for his foes. Only after his stroke did Michael seriously begin to regard Vincent as his successor. You could say that Michael's illness and Vincent's ascendency paralleled Vito's shooting and Michael's ascendency. But that'd be superficial, IMO. I think Michael's plan was for Vincent to take over the olive oil business and act as his pit bull and deterrent while he became "completely legitimate" through Immobiliare. And Vincent's quid pro quo was to give up Mary, whom Michael probably wanted to marry off to some wealthy, powerful and respectable European who could enhance his standing. He was using Vincent, all the way.