Is there any plausible way for something to be visible directly but invisible in a mirror?
You could say that there's an alternate universe, that's exactly like reality, except that there are no vampires in it; And mirrors have the curious property to reflect light from one alternate universe to the other. (You can hand wave towards quantum superposition or Schrödinger's Cat to make this sound more physical. If there could be a dead cat and a living cat superimposed in Schrödinger's box, why couldn't there be an undead vampire and a dead body superimposed in your world? Of course it's not really physically possible, but it sounds more plausible than "magic".)
This would help clear a lot of inconsistencies: For example, if is a dead person rises as a vampire, and you watch the scene through a mirror, you'd see a decaying corpse lying there (clothes, shadow and all), even after the vampire went elsewhere, because that's what would happen in a world without vampires. More precisely: That's what happens in the alternate universe without vampires. If you'd look in a mirror while a vampire picks up a glass, the glass would still be standing on the table in the mirror, because that's what happens in the alternate universe without vampires. (This also explains why the vampire's clothes aren't visible in the mirror, even if she changed after she rose from the dead. In the mirror, the clothes never left the shop.)
You'd have to have some effect that "moves" the glass and the clothes in the alternate universe later, when no one is looking in the mirror, to make sure the world in the mirror looks just as our world without vampires. (You could again hand-wave towards the effects of measurements in quantum mechanics, to make this sound plausible. The idea that something would disappear if you don't continuously look at it sounds strange, but in quantum physics, there are actually strange effects like that!)
Sidenote: If you use this in a story, you have to have at least 3 separate references to Through the Looking-Glass! (Because that's obviously where I've got the idea from.)