Michael made Vincent a don because the times were changing, Michaels ways of thinking were getting old, like his dads when he first took over in GF1.....outdated, Vincent was the future times were changing, and Vincent was a product of the changing times.....plus Michael knew that the only way he could really get out of the "business" would be to make Vincent the new Don......everyone would turn to him instead of Michael.....just like Don Vito wanted when he retired......you know, to grow his tomatos and peppers...lol.
I am sure that Anthony could have been a Don if he was dedicated to the cause, he may have even had that Corleone swagger too.....but Kay was too much an influence on him, plus after seeing his sister blown away, that may have changed his mind as well...