I don't think Michael had a soft spot for Vincent. He used and manipulated him, just as he did everyone else in his life:

Vincent's warning to Michael about Zasa, and Connie's complaint, shook Michael out of his Papal Knight reverie and made him aware that he still needed muscle after all. But first Michael tests Vincent: Michael throws him to the wolf (Zasa) by telling Zasa he has no quarrel with him and that Vincent is his to deal with; chiding Vincent in front of Zasa, then demanding that Vincent make his peace with Zasa. IMO, he was pushing Vincent to his limits, to see how willing Vincent was to be in Michael's service. When Vincent bites Zasa's ear, making the much more powerful Zasa his mortal enemy, Michael has Vincent where he wants him.

Vincent spends the rest of the film as Michael's ever-willing hachet man. With Immobiliare in his grasp, Michael anoints Vincen,then commands him to do the dirty work. The condition to be the new Don Corleone: he has to say goodbye to Mary.

I believe that if Michael had had his way, he'd have been "legitimate" as the head of Immobiliare, which he'd use to launder money. Mary would marry a respectable, truly legit, guy. Vincent would be his pit bull, scaring off would-be competitors and encroachers. Use, use, use.