I think it is intended that we think that Michael always regarded Vincent as family, at least in some sense.

Remember when he is talking to Vincent about his working for Zasa, and Michael says to Vincent something like "Mr. Joe Zasa now owns what used to be the Corleone family business, in New York. Out of the kindness of his heart he gave you a job in his family. Contrary to my advice, you took the job. I’d offered you something better, in the legitimate world. You turned me down.