Dracula: a Film Observation
In the 1979 film Dracula — namely, the scene where the vampiric Mina Van Helsing approaches her father Abraham in the mines under the graveyard — it is noted that Mina’s reflection is seen in the water. Two scenes later, when Count Dracula (Frank Langella) comes into the drawing room in Lucy’s home, Abraham is looking into the mirror, and points out that he did not see Dracula in the mirror.
The reason Mina does cast a reflection in the water (where she ordinarily should not have) is explainable by the fact that, just before her reflection became visible, Abraham had dropped a crucifix into the water, which had the effect of sanctifying the water — in other words, making it holy water.
Basically, there is one obscure detail of the superstitions about vampires; although they’re unable to cast a reflection in glass or polished metals, they do reflect in holy water, which is the only substance capable of showing vestigial remnants of the souls they lost to damnation when they died as living beings.