After seeing parts of GFIII for the millionth time on cable (whydon't they show I & II this often?) it is abundantly clear how brilliantly Michael manipulated Vincent and Connie by naming Vincent to be the Godfather here is what he accomplished:

1. He got Vincent to break it off with Mary.

2. He got deniablity from Connie and Vincent about his giving up the life of a criminal to become legitimate.

3. He assuaged Connie's desire to see Vincent succeed.

4. He also fooled himself a bit by thinking that he would be redeemed if he becme a "hands off" mafioso.

Of course had Mary not been shot, Vincent would have run the day to day crime operations in New York, as he said had to be done in the meeting with Don Tomassino, and Michael would be the esteemed head of Immobiliare, while still controlling Vincent. Put differently, he turned Connie and Vincent into "buffas."