...Michael was a cold calculating S.O.B. who manipulated just about everyone, and who bonded with no one. How then is his apparent soft spot for Vincent explained?...
Easy.
It was written into an already inferior story, thereby making the story even worse. Obviously, this seeming contradiction is the result of GF III being a bad movie!
To be fair though, also written in was Auntie Connie's prodding that Michael pay attention to Vincent, as she apparently saw him as the second coming of her father.
Apple