Vampires

I like the Castlevania explanation for why crosses work on vampires:
"Vampires are basically an evolved predator species, so their eyesight is pretty different to ours. Turns out that if you put a big geometric shape right up close in their field of vision, it confuses the shit out of their brains and, you know, makes them panic."
- This is similar to the explanation in Peter Watts absolutely brilliant novel Blindsight. The way vampire's eyes evolved had given them phenomenal eyesight, especially at night, but they go into seizure when trying to process intersecting right angles, a genetic abnormality that wasn't really relevant until humans started building structures and as people built more and more structures, the vampire threat waned, explaining their disappearance as well.
  
  I cannot stress enough how brillant that novel is. Not only has it modern vampires done right, but also AI done right and completely alien aliens done right.
- Don't forget to focus on your breath work.

Medical: vampires are just people affected by rabies.
People with rabies become adverse to strong odors (like garlic) or stimuli, like watching himself in the mirror. Biting of course is part of the symptoms and the disease is transmitted through bites.
After death it's common for these people to have blood oozing from their mouth, as if they bite someone just before.
And guess what nocturnal animal is carrying rabies ? Vampire bats.
- And holy water isn't their weakness, any water is because of hydrophobia.
  
  Okay Michael

Broke: Conditioning your vampire children to be immune to all religious symbols through exposure therapy
Woke: Seeing religious symbols in mundane objects everywhere because your vampire father thought exposure therapy was a good idea
Bespoke: Carpe Jugulum by Terry Pratchett
(I'm old and don't fully understand this format, but I like to join in)

Post-Bespoke: the most dangerous vampire is the modern day scientologist, whose faith is incomprehensible yet jealously guarded.
- Oh, you think you can defeat me? First you're going to need a Level 12 Clear Thetan, and that'll set you back $375,000. I know that sounds like a lot but we do offer 30 year, 0% financing, which makes the monthly payment surprisingly manageable.

Megaspoke: Vampires are vulnerable to all religions ever followed and constantly get flashbanged because they accidentally stepped on holy grounds of ancient civilizations
- that's why they're only in space
- 🌞

galaxy brain: vampires are just weak to crosses and liquids they come in touch with. christianity just leeched of the symbol's effectivness in fighting the undead and that's why it gotten popular.
- The Romans crucified Jesus to protect him from vampires.
  
  Thank you Pontius Pilate. First your saved Jesus and then you fixed my posture.
  
  twist: jesus was a vampire

Wokest: Sir Pterry, where vampires are weak to all religious symbols, but intentionally so, because death is so insignificant to them, the vulnerabilities that make them easy to stop also mean that no one truely makes their death more permanent.

Final boss: agnostic vampire
- Agnostic Vampire: Foolish hunter, I am far more powerful than you. You have no chance at success.
  Hunter: Yeah, I really don't know. You look really strong but I'm really strong too.
  Agnostic Vampire: Ahhh!!! Your uncertainty weakens me!
  Hunter: Oh wow, this feels like a trap. Now I'm even more uncertain.
  Agnostic Vampire: Ahhhhh!!!
- Alternative option: atheist redditor vampire
  
  Allergic to sunlight, story checks out.

The sun burns vampires because its the one true god
- Praise the sun!!!
- Hail to the Sun God!
  We know he's a fun god!
  Ra! Ra! Ra!
- Is that why I never see vampires in Vampire-Survivors?

There's recently been a surge of vampire media that retcons their weakness to religious artifacts as psychosomatic because it reminds them of their humanity. I really like that because it doesn't make any sense for someone to die and come back as a monster without something breaking in their psyche.
Basically like that meme that claims cartel members are weak to Dragon Ball paraphernalia.

So, if you get into the World of Darkness game Vampire: The Masquerade, this is at least in the ballpark.
Different clans of vampires have different vulnerabilities typically tied back to their origins. For the most part, this all comes out of the Semitic religions, because the OG Vampire is the Biblical Caine who was cursed into immortality for the crime of committing the First Murder. But the Kuei-jin - an eastern variant of the vampire myth - are more in line with the Buddhist / Animist ideas of reincarnation with their curse being a permanent exclusion from the Karmic Cycle. Different curse means different rules.
The Dresden Files series (which cribs heavily from WoD's Vampire) introduces similar ideas, with individuals of strong faith all enjoying supernatural power against vampires and other boogeymen that pays no respect to denomination. One of Dresden's on-again off-again allies is a trio of legendary swordsmen who use the nails from the crucifix to enchant their blades. But their actual faiths vary heavily - from traditional conservative Christianity to Buddhism to Communist-flavored Atheism to Work-a-holism to Geeky Sci-Fi fascination with the Star Wars Jedi. Only when a knight's faith wanes does the blade ever fail them. And the antagonists (demons, more often than vampires) are more fixated on tempting them to sin or disabusing them of their faith than of outright killing them.
- The last point was, IIRC, because killing a sword-bearer would just cause the sword to get passed to someone else and become a threat again. "Betraying" the sword (by turning away from one's faith) would actually make the weapon itself able to be damaged or destroyed, potentially ending the threat for good.
- You didn't mention True Faith!
  In WoD only people with True Faith could ward off vampires using a holy symbol or prayer; and seriously damage them, when on the offensive. Needless to say, having True Faith was fairly uncommon. This was back in the nineties, BTW.

If you are gonna rope us Trekkies into this, I’ll just ask if they want a salt tablet first. Then set my phaser to kill.

Bespoke: https://www.youtube.com/watch?v=HAom8ws8qp4

Does the Vulcan salute "Live long and prosper"
Vampire: hisssssssssss

In the bespoke vampire world, very few vampires are killed by sunlight, as these kinds nearly died out trying to continue worshipping the sun after being turned into vampires.

It's a bit .. much

Since in bespoke world vampires are living (in active sense) relic of old humanity. Bunch of vampire hunters are actually archaeologists in the field wanting to prove their hypotheses and once they find ancient ones they sample tissue, record mannerism and study as though they're coelacanth

Same vibes as the guy that "fought" a Lion with Toilet paper.
https://www.dailymotion.com/video/x2dqrsp
When did vampires become vulnerable to crosses?

There was a book I read where the exorcism failed when the Christian guy tried to do it, but worked when the best friend sang their favorite song. It was corny but sweet. (I might be remembering it slightly wrong)
- John dies at the end?
  
  Oh maybe. I read that a long time ago but don't remember the details anymore.

Woke: Haemovores from Doctor Who
The priest died because he had lost his faith. Haha. Anglicans. amirite?