I think it's like Niara said that D&D uses a lot of creatures from mythology and folklore around the world, but makes their very own version of it. They share the same name, but sometimes they are entirely different beings.
And also even in real world folklore, a lot of versions of these beings exists. Sometimes a stake to the heart is enough to kill a vampire, sometimes you need to decapitate it, or incinerate the body (Of course in real life people did this to corpses of people they suspected to have become vampires, or to prevent them from leaving their graves. And also a lot of other weird stuff we would call desecration today.)
So when we talk about something like a pixie or a vampire, we all already have different versions in mind.
Unless otherwise stated, I would expect that BG3 uses the D&D 5E Forgotten Realms versions, so that we are all on the same page.
And we don't have to worry we'll loose a rogue when we let Astarion carry our garlic 😄
