My guess is that the intern at CNN wrote “Former Vice President” and someone told them to edit it—and the edit got screwed up. Or they are so used to writing Former VP, it has become rote.
The people at these “sources” are not the “A” team. If you have social media coordinators in your company, you know what I mean.
I wouldn’t make a lot out of this. More likely the writer started to refer to him as “former Vice-President” then decided on “Democratic nominee” and just neglected to delete the word “former.” Been there and done that sort of thing too many times to not see it as a strong possibility.