I don’t like capitalizing Vampire Fact, but it’s possible that this is the name of a series of such “fun facts,” in which case it’s okay.
On the other hand, why is vampire capitalized elsewhere in this example? This is not a word that is normally capitalized, and there’s no reason for it to be.
It’s standard not to capitalize the first word following a colon if there’s not more than one sentence’s worth of words in the after-the-colon part. If there’s more than one sentence, you should capitalize that first after-the-colon word. (Yes, I know there’s some sort of obscure jargon for it. I don’t care. It is my strongly held belief that bogging writers and other sapient lifeforms down with obscure jargon is part of why few learn how to use punctuation correctly. Who cares what its name is, as long as you know what to do with it? You may not know an apositive from your Aunt Mildred’s best hat, but you can recognize a phrase that “renames” what came before it… which is what an apositive is. Thus, you can tell when that bit needs to be separated from the rest of the sentence with a comma or commas.)
Get rid of the em dashes and replace them with commas.
In the first sentence, change a mirror to mirrors (because obviously there’s more than one mirror involved in this). Also, since the beginning of the paragraph is about something that used to happen but no longer does, I’ve changed cannot to could not and added traditionally to smooth the transition.
Fun Vampire Fact: The reason vampires traditionally could not see their reflections in mirrors is because mirrors used to be backed with a reflective layer of silver, which, as the metal of purity, would not “interact” with vampires, who are the Devil’s work. However, mirrors have used aluminum as the reflective backing for many years now, and aluminum is not a “picky” metal at all, so vampires are able to see their reflections in modern mirrors.
(Although I strongly dislike vampires, I also strongly dislike discriminating against anyone on the basis of an allergy/aversions to a metal. Remember, there are normal humans who are allergic to nickel. Also, I think it would make more sense if they couldn’t see their reflections in polished gold, which is traditionally a “sun metal.” Silver is associated with the moon, it’s not as “pure” as gold because it can tarnish the way gold doesn’t, and silver is only a “holy” metal in the Current Era and among cultures of European origin because of its association with one of Christianity’s major myths. I’d be very interested to know if there were any “vampires don’t have reflections” legends from back when mirrors were usually made of bronze, and if so, what the explanation was then.)