What a coincidence, I'm ill right now. :P (Or not so much. It's that time of the year after all...) Anyway, it would be nice to have something a little bit more than just 'they run into each other'. xD Though a few images are coming in my mind as I think your idea over. A male noble running into a female vampire. But he doesn't know that yet. She's hot and seems rather lost in the world, while he's a bit bored with every day life. So he wants to keep her for one reason or another. Only soon he realises what she is. And she decides that living in relative comfort under someone's roof that can help her get adjusted to this new age would be better than finding her own way. And he decides that having a living mythical creature nearby, one that decided to not murder him for the time being, could be rather interesting.

Works with the male and female roles reversed as well of course.