The reason that vampires traditionally cannot see their reflections in a mirror is because mirrors used to be backed with a reflective layer of silver - which as metal of purity, would not "interact" with vampires, who are the devil's work.
However, modern mirror have used aluminium as their reflective backing for many years now - and aluminium is not a "picky" metal at all. So vampires are able to see their reflections in modern mirrors.
This also explains why vampires wouldn't have appeared in early photographs - black and white photography is achieved via gelatin silver.
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