Sonny's son? I think he wanted to protect him as much as he can because of the way things turned out with Sonny.

Probably would be a consigliere (think what Vito was to Michael) and try to keep him as safe as possible. Once Vincent chose that path, I doubt Michael would go on and bring him to the legitimate world after that - he had his children to think about as well. I don't think he'd want them to be too close, as in spending a lot of time together, but close enough that if his kids ever needed "support", Vincent would be there.