As stated before, mirrors are not a weakness, but an apotropaic . . . okay, even that is a stretch. At best a vampire confronted with a mirror will only be mildly uncomfortable. The real value in a mirror is that it can be used to reveal a vampire as the mythology goes that vampires do not cast a reflection.
The accepted reason for this is that vampires do not possess a soul. They are undead abominations lacking an immortal soul, which presumably left the body at the moment of vampiric conversion. This is the same general premise as a photograph stealing someone’s soul as it was generally believed that a reflection of any kind was an image of the soul inside someone.
It’s interesting that this idea persists into the modern day. For the early medieval time period (AKA Dark Ages) this line of thinking is to be expected—especially among the remote lands of Hungary and Romania (filled with Gypsy storytellers). But by the time of Bram Stoker’s Dracula in 1897, the superstitious thought should have never persisted in novelized form. Yet it did with Jonathan Harker noting that the count cast no reflection.
The concept of no reflection (whether or not the lack of a soul was the direct cause is not known) persisted in vampire literature and mythology through to the present day.