According to the Monster Manual, a variant Vampire spellcaster can cast the Mirror Image spell. If vampires aren't reflected in mirrors and other reflective surfaces would a mirror reveal reflections of just the magical mirror images or would it reflect nothing?
I sense some confusion in some of the other answers. I think I understand what you are asking, though.
A vampire can certainly cast mirror image and create illusory duplicates of itself. This is born out by the answer to this question, and solidified by the fact that the spell mirror image is in the Vampire variant statblock you quoted.
It is also stated in the description of vampires in the monster manual that they cast no shadows or reflections.
The rules are silent as to whether the illusory duplicates created by the spell can themselves be seen in a mirror. The rulebook has finite space, and play groups are expected to come up with rulings, where the rules are not explicit. This is one of those things that a DM will have to adjudicate. Some DMs might rule that an illusion of a creature that cannot be seen in a mirror cannot itself be seen in a mirror. Other might rule that the illusion does not have all of the supernatural properties of the original and thus it can be seen.
However, it doesn't matter.
The spell mirror image is not built to allow people to target an individual based on some deduction of which one is real. The spell details how attacks interact with the target.
Three illusory duplicates of yourself appear in your space. Until the spell ends, the duplicates move with you and mimic your actions, shifting position so it’s impossible to track which image is real.
The illusions are assumed to be constantly shifting. Even if there were seemingly four vampires in the area (the same five foot square), and a mirror did indeed only reflect three of them, this would give you no ability to target the real one any better. The tightly grouped images constantly shift through the duration. If you want to hit the vampire, you will need to use the targeting limitations imposed by the spell.