Stuck on the idea of vampires as a kind of reverse fae, or like someone's twisted, perverse attempt at moulding humans into fae.
They're repelled by liminal spaces.
A vampire could never enter fairyland, not just because they'd never be welcomed, but because most of the usual entry-ways are naturally barred to them.
They can't cross running water. They can't be seen in mirrors. They will wait forever at a crossroads, unable to pick a direction to go in. They can't even step over a thresh-hold unless there is absolutely no ambiguity about whether they are welcome inside.
They crave human blood, iron and salt, but are repelled by herbs and plants. They are supernaturally prevented from harming you unless the rules of hospitality have been invoked.
A fairy may replace your newborn child with something unnatural and ever-hungry. A vampire will do the same, but with your grandmother's corpse.
The fae are typically associated, even in stories where they're the bad guys, with flourishing and purity. Vampires, even in stories where they're the good guys, are typically associated with decay and corruption.
The fae turn ancient human burial mounds into fancy halls for their courts. Vampires take ancient human castles and let them grow mildewed and cobwebbed, exchanging the beds for coffins, turning them into burial places.
Fae don't tend to live among humans, but can generally pass for them with relative ease if they so choose. Vampires nearly always live among humans, but tend to find not revealing themselves a huge struggle.
I can't think of many stories I've read where fae and vampires even exist in the same universe, let alone ones where they actively interact. I feel like their enmity is almost more inevitable than that between vampires and werewolves, however.
The rivalry between vampires and werewolves is, essentially, the rivalry between two apex predator species who share a territory. (Even in stories where the werewolves aren't actually hunting humans.)
The vampires hate the werewolves because the werewolves interfere with their access to prey. The werewolves hate the vampires either because they consider themselves aligned with humans (the prey species), or because they are also predators and the vampires are competing with them.
By comparison, I think there's some story potential in the fae finding something genuinely creepy and uncanny valley about vampires.
They're immortal, like them, but also dead. They can be beautiful, like them, but that beauty is something they actively require humans to sustain. They like to inhabit beautiful and ancient ex-human dwellings, like them, but they actively work to make those places dark, damp and empty.
Fairies who are unflappable in the face of all sorts of Otherworldly monsters, can look an eldritch horror in the eye(s) without blinking, and have never been phased yet by any human, but will recoil from even the weakest vampire.
Vampires who hate fairies just as much, but in a more envious way. The way that the creature for whom immortality is a curse is bound to hate the creatures for whom immortality is an eternity of sunlight and laughter.
Maybe their touches burn each other. Maybe vampires can't stand physical contact with anything so alive and vital. Maybe immortal fairies become ill from too much exposure to the undead.
Maybe they fight over the human population when their territories overlap. The fairy need for servants and people to make deals with, competing with the vampire need for thralls and blood to drink.
Just… fairies and vampires. We need more stories about them interacting.
