Vampire Conspiracy Theory
It is believed that vampires can be killed with stakes or silver, don't have a reflection, are cold etc, etc. Is it not possible that beings of this kind could make this up so never to be found?
I suppose it is possible that vampires could spread false information about themselves, but there are a couple of reasons why I don't believe this is the case.
First of all, most information we have about vampires comes from sightings, witnesses, and investigations over thousands of years from hundreds of different cultures around the world. This is far back before it is conceivable that these cultures could pass information to each other in any kind of timely fashion. It is because the vampire myths are so widespread and so similar in various areas that this mythical creature still exists in our universal consciousness to this day.
Of course, you are right, this could all be vampires spreading these myths around in order to confuse us, but I don't believe that they have any good motivation to do so. Vampires have historically lived in secret. If anything, they would want us to believe that they don't exist at all. By going across the world spreading stories of themselves, they would have only increased their profile. There are only a few benefits to this and a ton of negatives. If I were trying to hide from the world, I'm not sure I would want to get a lot of people interested in me.
Then again, it could be the smartest thing to do. Hide the truth behind a myth so that if someone finds a body with bite marks on the neck, they will feel silly claiming that it was because of a creature that everyone knows is just a myth. It's like that line from The Usual Suspects: "The greatest trick the devil ever pulled was convincing the world he didn't exist."