At the time of their death, they become some sort of elemental, I suspect Air. That’s how they fly, turn to mist, etc. Their spirit, which is air, remains locked in the body when it should be going up or down.
This extreme unbalance to Air gives them power, but also weaknesses from the other humours they’re now cut off from.

They drink blood because that’s stealing the life of another being, a magical transfer rather than an altered digestive system. IIRC, blood was Water and Fire. They can’t produce it, so they have to take it.

They haven’t got an Earth of their own which would protect them from certain influences. They have to protect themselves with bags (or boots or cushions) holding an Earth they have a mystical attachment to.

The religious symbols brandished at a Vampire work as an attempt to restore the Natural Order, Elements in Balance. As Undead, they very much disorder the Balance. God dislikes disorder so pointing out a symbol of the Ultimate Lawgiver causes them to lose their artificial connections to Fire, Water and Earth. Pain.

Personally, I’ve never understood the Christain preoccupation with the Cross anway. If there’s anything i’d take as the ultimate symbol of the promise of Christainity, it would be the empty grave or the stone rolled back from the door. If i invented the vampire legend, the sight of a bloody figure nailed to a tree would give the monsters happy little hard-ons and make their mouthes water. Vampire lollipops would look…well, maybe that’s going too far.

I think there are a few different vampire legends in differenct countries. I have not looked up the Chinese lore, but I really should, because they’re done sketches where the vampires hop around and I wondered about that.