There's an event going on where a bunch of people post about vampires. You know who's a vampire?
This guy.
The original 'transformed' Castlevania Dracula, and the sixteen-bit homage to it introduced in Rondo of Blood.
Look at this fucking thing. This is a vampire.
In the Japanese manual to one of the NES games, it's proposed that Dracula struck a bargain with a forgotten evil god to resurrect its cult; when he reincarnates in the mid-2010s as a Japanese amnesiac, it's suggested that the cosmic force of Chaos has selected him to be a figure in direct and eternal opposition to god. It's never explained why this necessitates turning into a big freaky muppet-ghoul, though, or why other vampires can casually do it (presumably without being the earthly avatars of their own anti-deities), or why he has so many different 'true' forms.
(Although, interestingly, there IS a canonical explanation for why his pre-transformation human forms look different - Dracula 'resurrects' by incarnating his disembodied spirit into a suitable corpse, so it's literally physically not the same guy every time!)
Still, it's a really iconic and fun ability, and one that I think there's potential to use in RPGs. If Bram Stoker and Tracy Hickman can invent their own vampire powers for us to iterate upon and exploit, why can't Hitoshi Akamatsu? With Wizards of the Coast's push towards cinematic spectacle and climactic adventure bosses, it's shocking that Strahd von Zarovich hasn't turned into some kind of giant skeleton-snake-mosquito or discount Satan.
Things get especially hogwild on the Nintendo 64, where this lumbering thingamajig transports your hero from the foggy spires of his castle to some kind of wastelandish pocket dimension with a stormy yellow sky.
It's not as if you're lacking for options mechanically to represent whatever doofus your vampiric villain turns into - if your party is formidable enough to stand up to a bear (and they probably should be, if this is the kind of confrontation they're getting into), the most common bulky-bat-men Draculas bear a striking resemblance to the Type IV Demon or Nalfeshnee, and a one-off form that your villainous vampire turns into is the perfect opportunity to test out that esoteric random generator or gimmicky stat block from your favorite monster book that you're not sure you want to canonize as a whole species in your world. I also can't help but think of Nick Whelan's encounter tables, and the idea that "2 is always a dragon, 12 is always a wizard" - a single NPC who can turn into, effectively, both, gives a way to construct such a table without overcrowding the dungeon environment with independent populations.
Naturally the g-g-g-g-giiiiirl vampire gets a bit hornier of a transformation than Drac himself, but it's still an amusingly inexplicable combat form, and I think that tangled spine-hair is a fun image.
With my own weirdo lore about the Negative Energy Plane as a place of void, darkness, and unactuated potential, what if Vampires as Undead beings are less 'corpses' and more 'animated ideas'? If - as a certain frothing racist wrote - the oldest and most powerful emotion of all sapient beings is fear, then a vampire might be a bodiless consciousness from the Negative Plane, parasitically attaching itself to a creature's fear of ambiguous spookums that live in the dark to take on semi-corporeal form. Feeding on blood without completely killing and exsanguinating could be a way to maintain or strengthen this connection - a vampire's immortal brides could be captives it perpetually feeds upon every so often, not because it has any actual dietary or caloric requirements but to keep them in a state of awe and fear at the terrible hungers of the vampyr.
Direct confrontation, then - or unshakable religious conviction, or the pure clear light of the sun - could force a vampire to stop being nebulous and take on a definitive form - still fearsome, yes, but ultimately knowable and slayable. Perhaps the misty form taken on HP depletion of the classic RPG vampire is even an instance of this - a vampire whose power was so lacking that it could not 'resolve' into anything, and dissipated into bodilessness, to either return to darkness where something else horrible might lurk or be banished forever back into the realm of potentia.