I've noticed that some vampires (Hert, Hern, Sybille Stentor) have glitched faces. Checking uesp, it seems that the vampires with this glitch are ones who had their appearance updated in Dawnguard.
http://i.imgur.com/AQZud.jpg

The only facetexture changing mod I use is No More Blocky Faces. I uninstalled that, but the problem persisted.
It's not the world's biggest bug, but it does somewhat annoy me. Anyone got a clue what can cause this so that I can track the problem down?

It's the USKP. The UDGP will fix this eventually.