Vampires are aliens.
I mean, bear with me for a second here, because I'm recycling the idea of a show (Legends of Tomorrow) which utterly sucks, especially when it comes to writing and consistency.
But imagine a scenario where a small meteorite strikes Earth, and on it is some sort of alien element or technology that transforms the people around it into vampires.
Given the nature of vampirism/the spreading of the transformation, you could introduce it as an alien virus or a form of alien takeover even.
You could also keep the meteorite aspect alive by giving the meteorite special properties when it interacts with vampires again (weakens their powers, kills them, makes them more powerful, etc.).
6This is kind of broad as they could come from anywhere, a God got bored decided to make a creature that would pray on its on species. Aliens were doing genetic modification to a host and something went wrong and the made a vamp. It could be evolution and the only source decent nutrients are from blood. They crawled out of the grond and clain they are the rightful heirs to the earth .... Any of these could an anser in its own right, narrow down what you are after and we will do what we can to help – Mr.Burns – 2016-06-13T13:08:08.880
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Welcome to the site. Please note that the Worldbuilding Stack Exchange is dedicated to providing detailed answers to specific problems, rather than a discussion forum. While we do entertain questions about vampires, their origins are whatever you choose for them to be. See Risk Factors for more information about our scope for questions.
– Frostfyre – 2016-06-13T13:08:21.030Welcome to the site. In addition to the comments above I will point you to our [help] and once you hit 20 rep feel free to visit and discuss things like this in [chat] – James – 2016-06-13T13:52:36.590
It's best to not ask questions looking for ideas. If you put a little thought in, you could probably come up with a list of causes, and then ask a question about the one you like most. Here are a few examples to get you started: A curse. Possession by other worldly forces. A viral mutation. A random genetic mutation. Aliens. A species that evolved separately and has lived along side us, but with few numbers (like a normal predator:prey ratio). Mental disease (normal people otherwise). etc. I hope some of these ideas might help. – AndyD273 – 2016-06-13T15:25:35.213