Malfoy here would be fascinated with you being a vampire, he hasn't come across a vampire ever in his life before. But Draco wouldn't really care that much if you're a vampire or a werewolf or whatever it is that you might be. He loves you for who you are and your personality, after all, your personality is what made him take notice of you. So when he takes notice of you, that's when you'll have to kiss your freedom goodbye since he'll start becoming obsessed with you and you'll be occupying his thoughts constantly, every single moment of the day much to his annoyance. He doesn't understand why he's thinking about you so much 

So, like every other douchebag character in movies and shows where they bully the person they like since they just want their attention, unfortunately Draco's like that as well. It really shouldn't come off as a surprise. He'll go a bit easy on you if you're in Slytherin though but if you're in any other house especially Gryffindor, he'll keep teasing you constantly but never harm you physically or hex you or anything. He does want you to be in a perfect state, he doesn't like to see you get hurt. And he doesn't like seeing you spending too much time with those riff raffs like Saint Potter, the Blood traitor Weasleys and the Mudblood Granger, they're trying to steal you away from him! So, he won't really go easy on them either. Or anyone else really, whoever he sees you spending too much with other than him 

How he comes to know you're a vampire is an interesting case. One day during Potions class, Professor Snape mentioned something about vampires and when someone said that vampires were dangerous and out of control, you immediately defended the vampire species, well, it's only natural to do so since you ARE a vampire after all. So, you sassed the moron whoever said that comment and ended up having 15 points taken away from your house for disturbing professor Snape's class but you felt it was worth it. Malfoy on the other hand, his eyes were narrowed to slits, exactly like a snake's. Why were you so eager to defend vampires? Could you have some obsession or fondness for them? Which also made him a bit jealous, being the absolutely unreasonable ferret that he is since you should be thinking only of HIM and no one else! Could you have some magical vampire friend that he doesn't know about? He intends to find out and get to the bottom of this mystery 

The second instance Malfoy observed your strange behavior was during the class of Transfiguration. Neville had injured himself trying to cast some charm on his parrot and there was a bit of blood oozing out from his wound. Your eyes immediately turned red and you felt your fangs sharpen as your senses became more alert. You started getting the feeling of pinning Neville down to get his blood but you managed to control yourself and unfortunately for you, your eyes connected with Draco's and he smirked. He finally got to know that you were a vampire, after all his father was friends with a few vampire so of course he'd recognize a vampire when he'd see one. After class was done, you knew Malfoy was going to tell everyone and end up doing something or the other to get you in trouble so you had to plead with him not to tell anyone about you 

Malfoy had that irritating and annoying smirk on his face as always and seeing that just made you want to slap that right off his face. He pulled you into an abandoned classroom and you gritted your teeth as you spoke "Malfoy, don't you dare tell anyone about this else I swear to Merlin I will suck all the blood out of you'' and he just replied "You're cute when you're angry. How adorable. And I won't, provided you do something for me'' and you didn't like the tone of how he said it. You had a bad feeling and you replied "What do you want?" and when he asked you to go out with him, you almost doubled over. The Slytherin Prince, Ferret Boy here wanted you to go out with him? 

"Malfoy, you can go out with any person you want, why do you want me? You're deliberately doing this just because I'm a vampire, is that it?" you demanded and he grabbed your hand and pulled you closer to him as he responded. "Because I want to. You're mine'' and from that day onwards, he never let you out of his sight. He kept clinging onto you like some koala and every time it was the blood moon, you'd try to control your urges since you didn't want to depend on Draco for help, but he insisted on helping you and you sunk your fangs into his neck as he caressed your head lovingly and called you his sweet little vampire. You were never going to be free from him