Why doesn't (2) make sense? They might indeed be dimmer.

But then vampires are visible in mirrors, which violates the premise of the whole thing! Being visible as a patch of dimness is just as bad as being visible as an evil-looking fanged person, from the point of view of a vampire trying to sneak up behind a maiden admiring herself in her bedside mirror. Either way, she's going to notice something fishy is up, turn round, scream, and thereby summon stake-wielding security guards.

Hmm, good point. Perhaps there would have to be a split in energy such that the S photons (if those are the ones that are reflected by the vampire but not then by the mirror) get most of the energy and the R photons come out with hardly any, like a gentle infrared signal. Ooh, in fact, you could play with the energy split in terms of wavelength and come up with some nice stuff. Because I needed something else to do this weekend like a hole in the head...

Well (he says, noticing he has a vampire-duck userpic) the amount by which the vampire was dimmer viewed directly would have to be the complement of the amount by which the background was dimmer viewed in a mirror. One or the other would have to be at least 50% dimmer than usual, which would surely stick out like a sore thumb?