So, the underlying question here is: how much knowledge about vampires do adventurers or regular people in the forgotten realms have? Do they know their weaknesses (sunlight, running water, stake in the heart…), their reproduction mechanics, their feeding habits?
The reason I am asking such question is that I am currently running Curse of Strahd (5e), and the characters in my party were brought to Barovia from the Forgotten Realms. I was wondering how much they should know about vampires before going to the adventure. Being 1st level adventurers, they obviously never faced a vampire, but how likely is that they read books about it, heard about it in random tavern chitchats or obtained such knowledge in some way?
As an example of why this question matters: would it be obvious that a sun blade (especially the Sun Sword) is extremely useful against vampires (especially the sunlight property)? Would they think about bringing him or any other vampire to a river?
Note: To clarify, my intention is not, in any way, to stop players from using such knowledge. I am running CoS with a very open mind on metagaming anyway. I want to understand/know this because the adventure has a handful of situations where the characters are given such pieces of information, e.g. when the Tome of Strahd tells them he is affraid of the Sunsword. However, that knowledge is quite meaningless if the characters already knew/know that vampires, in general, are weak to sun blades, any way, and borderline useless if the characters already possess the Sunsword (so it doesn't even tell them the sword exists). So I was hoping that I could justify – at least to the players – that their characters are actually learning something new, although it is obvious to the players themselves.
Best Answer
Almost certainly a varying amount is known by different people. It may help to work this out with your players in advance of their finding any clues or use dice to determine it, rather than make an arbitrary call on your own
A few thoughts leading up to the answer:
Applicable knowledge is wildly uneven in the world. Not only are there almost certainly people in the real world who don't know all of the standard vampire tropes, there are also disagreements over what counts as a trope and what doesn't (is Dracula the only canonical source of information, or True Blood, or monster stories about vampires from antiquity? Some combination?).
Regarding useful knowledge of things which actually exist demonstrates similar disparities. A lot of people know first aid procedures, but many do not-- it's entirely possible that a person could have a heart attack in the middle of a crowd of people, and none of those people know CPR. Many people go camping but wouldn't know what to do if they run into dangerous wildlife like wolves, bears, or mountain lions. So even if a threat is known to exist, knowing it is one you're likely to encounter in the near future doesn't guarantee that a person will know how to deal with it.
While there are certainly ways to transmit information quickly in most D&D settings, there is probably not an equivalent to the internet or blockbuster fictions. Living in Baldur's Gate probably gives a person access to a greater variety of information about the world than living in a pastoral village does. Whatever the general level of vampire knowledge is, that doesn't have to mean much for a specific group of characters.
Taking these two points together, unless vampires are a more or less daily threat in the Forgotten Realms, there is little reason to assume any given character has much information about them, accurate or otherwise. Even if they know some information, unless your players' characters are specialists in a relevant area they are not likely to know the entire Monster Manual entry for a vampire.
From a TTRPG-with-a-GM perspective, consider:
How would you feel about arbitrarily changing some of these details to simulate lack of PC knowledge in your players? If you switch a vampire's weaknesses from a wooden stake to an iron stake, sunlight to moonlight, or running water to unbroken lines of salt, would you consider those unfair? Would you tell your players about some or all of the new weaknesses?
The more reluctant you might be to make such changes, the more your game effectively treats information on vampires as common knowledge. It's not a problem either way, but if the game experience you want leans towards a particular answer to the question of how common that knowledge is, you may as well simply assert that answer.
How significant do you want the clues to be? Information can be a flimsy reward when the players likely already have it via cultural osmosis (including Curse of Strahd, one of the most famous D&D modules ever!). But it doesn't have to be that way.
Minor tweaks can make the clues far more relevant, or even necessary, than simply providing information the players already have. Maybe there is a (homebrewed) special condition under which the Sun Sword frightens Strahd, and that condition is totally separate from the common vampire mythos. The players have to get the clue from the tome to get the advantage-- skipping steps because they "already know" doesn't work.
My personal favorite: introduce uncertainty, and uncertainty about the uncertainty, to keep your players guessing. In several situations I've told my players things like
This dramatically increases the value of finding information in-game, doesn't require major scenario changes, and can be described in within-game terms pretty easily ("You don't really believe that, do you? That's just a folktale!"). My players also seemed to be excited by the mystery, and the fact that they didn't know how many details I'd changed (or how dramatically I'd changed them) meant that even finding something that matches their expectations didn't make things predictable for them. As long as you warn them in advance about the potential changes, the players won't feel blindsided if and when their assumptions aren't borne out.