The rules, in this case are pretty clear. The spell never mentions mirrors or reflections, it “summons illusory duplicates." Spell names aren’t indicative of their mechanical effect. See Chill Touch.
There’s also vampire wizard statblock that has Mirror Image on its spell list.
It would be funny if the spell just failed though.
I suppose it depends on whether applying the mirror to a vampire with mirror image counts as an “attack.” It is an action that functions similarly to a divination spell. If the Vampire casts Mirror Image, and then you try to “target” the vampire with the mirror, there’s a chance you target one of the mirror images, and it would be visible in the mirror.
Vampires also don’t cast a shadow, and Mirror Images are identical images, so none of them would cast a shadow. So while the mirror trick might not work against a vampire who has cast Mirror Image, you would be able to use any light source, including ambient light, to check if any of the images are casting a shadow.
I don’t I can make a good case for “looking in a mirror” to use the “attack” rules. If it’s an attack you’d have to make an attack roll (if you’re not making an attack roll, it’s not an attack.)
Likely, if there was a d20 roll involved, it’d be a perception check.
You’re not wrong about the RAW, but it’s a point of contention for the RAI on how Mirror Image affects non-attacks that target the caster.
My feeling is that using a mirror on an enemy is like an attack for the purposes of interpreting the RAI on Mirror Image. Mirror Image only specifies what happens if the caster is attacked (the false images have a chance of being targeted instead) but there are Divination spells and some offensive spells like Magic Missile that require no attack roll. Some people interpret that to mean that those spells ignore Mirror Image, but that argument doesn’t track for me.
Anything that requires targeting an individual by sight should be affected, even if it isn’t technically an “attack” per the rules. It should be up to the DM to decide. The description of Mirror Image makes it clear that it confuses anyone who can see it, making it difficult to determine which image is the real one. It describes specifically how to calculate attacks, but it also doesn’t specifically say that non-attacks are unaffected.
If you use that interpretation, it would suggest that pointing a mirror is like targeting the character who has cast Mirror Image. You would have to roll a d20 and compare that to 6, 8, or 10 depending on the number of mirror images. Perception can’t see through Mirror Image.
You might see no reflection (which would confirm they don’t cast a reflection), or you might see the false image’s reflection (which would be ambiguous).
This all assumes you’re using a steel mirror or similar item to see the reflection. If there is a mirror in the room, you would be able to see more of the room, and presumably you’d be able to tell if the caster had a reflection by comparing the number of reflections you see to the number of duplicates in the room with you.
I’m pretty sure that the rules as intended for this one are that it just affects attacks, I’m afraid. Sage Advice repeatedly argues that the intention of wordings like this is that they don’t extend to other effects.
Of course, it’s perfectly fine to run your table different to sage advice. There’s a lot of stuff in there that I think is rather silly, or bad design.
Were you able to see the whole space, you should be able to see the illusory duplicates fine (they’re not “vampires” mechanically), but not the real vampire, so I’m in full agreement there.
Mirror image specifies that it blocks attacks that are targeted at the caster. Magic missile has no “attack” component, it automatically hits. Mirror image also ignores AoE damage. The specific wording is “A duplicate can be destroyed only by an attack that hits it. It ignores all other damage and effects.”
We just had this discussion at our table on Saturday.