One thing I've always loved about vampire stories is that there are no hard rules. Vampires can be whatever the author wants them to be as long as they're internally consistent within the story. This makes them extremely versatile monsters that can be constantly re-invented and none of them are "wrong".
For example, sometimes all it takes to turn into a vampire is to be bitten by one. And sometimes you have to drink the vampire's blood to turn. Or, sometimes a vampire can just drain your energy without transferring any blood at all.
Of course, the rules can also get a bit ridiculous. Maybe the vampire is required to obsessively count grains of rice first. Or maybe they're required to hop around with their arms out (the chinese Jiangshi). Maybe they can turn into a wolf, a bat, or... fog.
So what's the most ridiculous rule or type of vampire you've come across?
(For the image on this post I wanted the most ridiculous picture of a Jiangshi I could find. I went with a shot from Robo Vampire which has a really fun RiffTrax. The only other Jiangshi I'm aware of is in the NES game Phantom Fighter... because I've never watched any of the Mr. Vampire movies)
The counting objects thing was funny when they used it in that X-Files episode. Basically trying to make a lot of the goofy lore seem plausible through science, so the vampires all had OCD and they slow one down by just dropping a box of pens or something that the vampire had to stop and pickup while counting. lol
The other one I've only seen used twice (Buffy the Vampire Slayer the movie and in Soul Reaver) is that vampires can't cross running water. In Buffy, she stops a bunch of vampires chasing her by turning on a hose which prevented them from crossing the stream of running water along the ground.
I don't remember that episode, but I would think the vampires could have just stepped over the hose. Unless water flowing inside a hose also qualifies. So follow the hose to the tap and shut it off. But presumably the principle only applies to running water that's out in the open, otherwise vampires would be unable to move around in cities or towns because they're honeycombed with water pipes, or through a lot of natural areas because of underground streams. Maybe there's a range limit.