Chm... I wanted to suggest moving this threat into Vampirism... too late I guess. But you should be aware that there is section of the forum dedicated to vampirism. (my favorite.
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Combination of nobility and vampirism is modern thing, that started really with generation of writers that included Bram Stocker. You might know the novel Dracula, but there were more, The Vampire, Carmilla, Clarimonde... those are one of the oldest mentions of vampires among the nobles.
But in Middleages vampire was revenant, a living corpse that people feared. Simply undead that was hunting the living, something that was wrong. And claiming a noble to be undead was something none would dare. The vampires were mater of common folk, mostly connected with fear and superstitions of magic and evil. No one wanted to be a vampire when everyone wanted to go to heaven and walking dead were condemed, lost souls destined to hell... in their minds. It was very different society from today. That is why you cannot find any historical evidence of vampirism among nobles. Even considering that back then would be heresy, the nobles were chosen to lead by god. That's what they believed.
So, among modern people many have heard of the idea, but it has no real historical background. Elizabeth Bathory was a noble woman who was a widow who refused to marry and wanted to be independent. And so they started accusing her of witchcraft to get to her vast wealth. Just one example. Dracula wasn't vampire either. the tales of him drinking blood were made by his enemies and haters, he was simply man and politician of his age. When we stick to hard facts, we have to admit this. The nobles are no longer as they used to be, and so they became legendary... and their legends got mixed with legend of vampires at some point. That is the bottom of it.
