Mirrors have been seen to reveal a person’s spiritual double (the soul). When a person saw themself in the mirror, they received confirmation that they had a soul. Thus, if there was no reflection, it was thought the person did not have a soul, such as a vampire.
Mirrors forced vampires to confront their nature of existence as an undead, which is why many vampires do not like mirrors.
Mirrors were more common in vampire fiction than in real vampire lore.
Although, throughout many parts of Europe, when a member of the household died, the remaining family would turn the mirrors towards the wall. This prevented the spirit from reflecting itself and returning to its reanimate body. If a vampire walked by a mirror, there won’t be a reflection because the spirit is wandering and never present in the body.
In parts of central and eastern Europe, it’s important to pour out all standing water from containers after a death has occurred because water can create a reflection of the soul’s image. In Romania, all containers of water are covered at night because the spirit wandering around at night may fall in and drown. But, in Macedonia, a bucket of water was left by the grave to create a barrier and ensure that a bad spirit would be entrapped in the grave if it tried to escape.
Most of these water customs were developed pre-industrial Europe, when mirror were scarce, and those that were around were of poor quality and produced a distorted reflection.
When it comes to fiction, Bram Stoker popularized the idea that vampires could not be seen in mirrors. Anne Rice reinterpreted the myth and said that because vampires lived in the same universe as mortals, they had to conform to the same physical laws, which included optics and could be seen in mirrors.