Well, for one thing, we are incapable of extending the sound d? , while ? can be extended as long as your breath holds out: that is, you can pronounce 'vision' as vi??????n. It is good to practice it like that.
So, If d? can be extending like ? in vision vi??????ion, it is not d? sound anymore? (because it cannot be extended?)-- That's right.
Then, is d? not d+? two sounds? ( because only ? can be extended..)-- That's right, as far as the practical physical production is concerned. I am not at all a speech physiologist, however. I am just telling you what I use to teach my students