Can a Vampire Lord buy a house if s/he has not been made a thane before become a VL? I can buy Breezehome, but I've finished the quests that should get me the option to become a thane of Riften, and done a lot of different quest for Riften citizens and still the steward won't tell me to talk to the Jarl about becoming a Thane. There's no option to ask the Jarl of Markarth if there's anything I can do for him. If I ask about buying a house he tells me I'm an outsider, and to talk to his Steward for anything further. His steward doesn't have anything to say except the usual pvssyr--no dialog options.

I've bought houses before for other characters and not had this problem. The only difference is that I'm now a Vampire Lord.

So is this a bug, a feature, or something I'm missing?

Riften has a unique method of becoming Thane. Try speaking to an Argonian woman called Wujeeta.

In Markarth, you must be level 20 before you can become Thane.

I've already done the Skooma quests--given the Argonian her potion, cleared out the warehouse, cleared out the cavern, told the Jarl and gotten thanked.

I'm level 27 at the moment.

The Jarl of Markarth and his Steward both had options to look for work the next time I went back, but still no option to "serve the Jarl" to get the shield quest.

Hmmm...
So technically it's not Dawnguard and House Ownership.
The question is:
Vampire Lord and House Ownership?
On behalf of the local Ratepayers Association, I wish to point out that Vampire Lords are unacceptable neighbors, as per chapter 9 subsection 4 paragraph 7E of the municipal code...
So get yerself back up to your fabulous castle in the north!
Or what?
It wouldn't surprise me actually...