The details of the dinner (as I have read) are really vague. As you can imagine, these nicknames that rulers have were made according to their fame. So he earned "The Impaler" for been this (in)famous.
What I think is that he just liked to screw around with his enemies while he (and only he) had dinner. But yeah, these details are really awkward...
RE: Vampires: The Beginning of a Myth & its Scientifical Explanation