Getting Bolts as Vampire

It is not possible to use any of the Dawnguard vendors as long as you are a vampire yourself. (They'll all point you at the Morthal quest to cure your vampirism).

The good news, is that once you finish the Dawnguard Main Quest, you can ask Serana to turn you into a vampire lord whenever you want!

So as long as you have a supply of black soul gems to return yourself to being a mere man (or mer), you can bounce back as much as you like between vampirism and normalcy (as the cure vampirism quest is, in fact, repeatable).

So to answer your question, no, but you can continue to be a vampire lord with a little extra effort on your part.

If you're playing on PC, you can also access the console to acquire bolts that way.