I think it’s just the operation of the “Rule of Cool” as applied to artsy scenes.
“Mina Van Helsing” makes her first on-screen undead appearance as a reflection in a pool of disturbed water. The image is just a confused pattern of white reflections (anomalous, since we’re in a dark underground mine tunnel) that resolve themselves, as the water becomes still, as the reflection of the undead Mina. It’s as if the ghost is slowly materializing (kinda like the way the flames assemble themselves into a Pillar of Fire that writes the Ten Commandments in The Ten Commandments). Only it’s a naturalistic way of making it happen, unlike using anything as crass and crude as special effects. The reflection in the water lets the Vampire Mina appear from nowhere, although even within the events of the movie, she was actually standing some distance away and simply came close enough to be reflected.
It does, of course, violate the Rule of Vampires that they have no reflections, which derives ultimately from their lack of souls (so it doesn’t matter that it’s a reflection in water, not a glass mirror). But director John Badham, I maintain, didn’t care. The neat shot and its symbolic imagery trumped consistency.
Dracula’s smashing of the mirror shortly after that derives from an older tradition, the scene in the Deane/Balderston stage play. This, in turn, derives from the shaving scene in the novel where Drac fails to appear in the mirror. So you had two trajectories – the “Dracula doesn’t appear in the mirror (because he has no soul)” meme that you can trace back to the source and was a long-standing part of the story, alongside the "Mina appearing from the Aether’ scene that’s newfangled, and may not even have been in the script. Badham left them both in. Even if he cared about the diusc repancy, pulling the mirror scene out removes another dramatic moment – Dracula smashing the mirror – and that would also leave a gaping hole in the dramatic Dracula-Van Helsing standoff scene.
So he kept both scenes, despite their fundamental contradiction. of such are the movies of Heaven.