There are few things you need to know about the Potions Master of Hogwarts. He's a simple academic man, with a mask sewn from mithril, silk and rubber to fool even the best and a simple minded determination to pursue the object of his fascination.
This interesting 'thing' just happened to be a new type of vampire he had never heard of before. And this fact leads us to his private lab miles down in the heart of Earth, where nobody could see, hear or smell you if they tried.
He had to travel to the US to capture his victim... Khm... Test subject... Khm... Guest, but with the help of a few nice native teen, or were they young adults, he managed to petrify one and was now looking down at him hungrily like a kid before tearing into his Christmas present, shredding the paper into small tufts, not recognizable anymore.
His guest was pale, like all known vampires, he started narrating in his mind.
Trying to pull the lower lip down, to inspect the fangs on the other hand, turned out to be difficult. The skin he touched was cold as ice and hard like granite. Obviously dead, not counting the frantically flicking eyes, and the pretentious way it seemed to mimic breathing. He almost snorted at that one.
Flicking out his wand, he guided the aforementioned lip down, confirming the existence of the eye teeth, apparently leaking venom. Now That was an opportunity he couldn't pass up. He gathered a vial and filled it with the dark green sludge. He would test it, and if he saw the potential, maybe he could even experiment with it in his potions.
After carefully placing the vial in its new place, he absentmindedly let the lip spring back to its place, closing the mouth.
The subject... Guest seemed to be fairly handsome, most likely due to its condition of being a vampire. He reminded Snape of the flesh eating plants that beckon their prey with sweet smell and nice look, just to shut their cage the second the food is inside. It reminded him of the deceptions of different animals and bugs. How... Slytherin of Nature.
The small conjured light showed no diverse affect, only glittering. Snape took a deep, slow breath not to crack or retch at the sight. So the vampire either had another powder coating him, or it was literally crystallized. Fun and games, and more things to test. He didn't waste much time finding a small bowl, and starting to scratch the skin down the guest's skin, right from his cheek and nose.
He went on, testing samples, and theories, going from garlic to right out slamming a stake through the subject's chest, before waiting for reaction. He was forced to cancel the petrification by this time to see if he died, but cautious enough to bind it by other means. When he chapped a finger off and it started moving, he was slightly unnerved. His guest was silent, aside from the growling all this time, but when he took out a match stick, he started babbling and begging.
Severus got every information he ever wished for on the subject.
That didn't mean he was going to let his source of fascination, the provider of new ingredients and his new way of entertainment and stress releaser go. At all.
It was a gleeful ex-Death Eater, now spy that went to bed late that night. He enlisted a few house elves to watch his vampire, Edward or something, at all times and prevent escaping, and provide food - along with covering up his little secret.
Meanwhile, in Washington, Forks, was a very confused girl, wondering if that one meeting could really chase a guy so far as to run away from home and what did she do, or what the hell it was that frightened the guy so much?