the original explination was that mirrors reflected the soul, and since vampires didn't have a soul there was nothing to reflect. If you don't like that you can still do something similar by saying the magic is no in the vampire, but the mirror. Perhaps certain mirrors are specially enchanted as vampire-detectors. Or perhaps it's simply a basic principle of magic that mirrors behave that way.

Many worlds have varying degrees of clap you hand if you believe style magic, where magic works the way it does simply because everyone expects it to. If you go with a system like that then you can simply say that because people expect the mirrors to not reflect vampires that's how they work.

If you only want mirrors to reveal vampires, and aren't committed to how they do it, one option I personally like is that the mirror still reflects the vampire, but it reflects a rotting and disgusting corpse. The vampire actually is undead and rotting, but he uses some form of magical suggestion to make you see a beautiful creatures (and presumabely not smell it); however, he can't use the same powers to keeping you from seeing his real reflection. This gives some fun potential to have someone be disgusted at the real sight image of someone that looked like a sex-god a moment earlier to them; which helps bring a very viscerale sense of how 'wrong' a vampire really is. Insert joke at expense of twilight sparkling vampires here

You could take this one step further by suggesting that if a vampire is very skilled and focused he may be able to control your perception of his reflection in a mirror as well, but it's very difficult and he has to do it for each person. Thus he may be able to occasionally appear in a reflection if he knows he has to walk in front of a mirror where someone may notice; however, he can't do the trick if caught by surprise, if he doesn't know of someone who might see the reflection (he fails to consciously control what that one sees), or there are many people that would see the reflection (he can't consciously screen his reflection for that many at once). Due to the difficulty of doing this though most avoid mirrors entirely, for fear they will not be able to control what everyone sees, and perhaps only the oldest and most powerful vampires can do it (thus you can show a vampires strength by having him casually walk in front of a mirror). This option gives you a little more control over when a vampire's glamor may break, at any point one person may see his reflection is wrong while other's don't etc.

The original explanation was that mirrors reflected the soul, and since vampires didn't have a soul there was nothing to reflect. If you don't like that you can still do something similar by saying the magic is no in the vampire, but the mirror. Perhaps certain mirrors are specially enchanted as vampire-detectors. Or perhaps it's simply a basic principle of magic that mirrors behave that way.

Many worlds have varying degrees of clap you hand if you believe style magic, where magic works the way it does simply because everyone expects it to. If you go with a system like that then you can simply say that because people expect the mirrors to not reflect vampires that's how they work.

If you only want mirrors to reveal vampires, and aren't committed to how they do it, one option I personally like is that the mirror still reflects the vampire, but it reflects a rotting and disgusting corpse. The vampire actually is undead and rotting, but he uses some form of magical suggestion to make you see a beautiful creatures (and presumptively not smell it); however, he can't use the same powers to keeping you from seeing his real reflection. This gives some fun potential to have someone be disgusted when they realize what the elegant or even sexy looking vampire they had been talking to really looks like; which helps bring a very viscerale sense of how wrong a vampire really is. Insert joke at expense of twilight sparkling vampires here

You could take this one step further by suggesting that if a vampire is very skilled and focused he may be able to control your perception of his reflection in a mirror as well, but it's very difficult and he has to do it for each person who sees the reflection. Thus he may be able to occasionally appear in a reflection if he knows he has to walk in front of a mirror where someone may notice; however, he can't do the trick if caught by surprise, if he doesn't know of someone who might see the reflection (he fails to consciously control what that one sees), or there are many people that would see the reflection (he can't consciously screen his reflection for that many at once). Perhaps when he is doing this it is such a strain he may be distracted or act odd in a way that may give subtler hints that something is wrong. Due to the difficulty of doing this though most avoid mirrors entirely, for fear they will not be able to control what everyone sees, and perhaps only the oldest and most powerful vampires can do it (thus you can show a vampires strength by having him casually walk in front of a mirror). This option gives you a little more control over when a vampire's glamor may break, at any point one person may see his reflection is wrong while others don't etc.