...All of this is pretty sketchy though, it's definitely pushing the limits of what these spells or abilities are intended for, and "having your image move around inside you perfectly so nobody notices" would require an extreme level of control that I imagine most, if not all casters wouldn't have.

If we're trying to design an NPC vampire to foil the mirror check, it's probably better to have them use a custom spell or magic item that's explicitly for this purpose than to try and repurpose an existing spell to do it. "This amulet creates an illusory duplicate of you that fills the same space as your body. When you turn invisible, the illusion remains visible and moves in a random direction away from you" for example.

This approach has a lot of advantages I think.

1. If you let NPCs do it, then PCs can do it.
2. It doesn't feel like you're creatively abusing the rules to give NPCs an advantage
3. When the PCs beat the vampire and get the treasure they get a lightbulb moment when they realize what happened
4. The players get an interesting niche treasure out of it they can come up with clever uses for later.