1 Adam: This is where our mirror comes in handy.
1 Jamie: How so?
2 Adam: We can use a mirror to safely see things!
3 Jamie: That only works if there’s light. Mirrors are useless in the dark. Also for seeing vampires, remember.
4 Adam: Are you sure it works that way?
4 Man (possibly a vampire): Good eee-vuh-ning.

Mirrors are traditionally used to look at things that would be deadly to look at directly, such as the Gorgon Medusa from Greek myth, or a basilisk or cockatrice.

This carries over into roleplaying games such as Dungeons & Dragons, where such monsters have dangerous gaze attacks. So when such monsters are suspected, adventurers may use a mirror look around a corner, before advancing if the coast is clear.

But if the creature around the corner is a vampire...