Well, the first thing to note is that it is generally accepted that vampires are purely mythical creatures, that is to say they don't actually exist - hence it is actually immpossible to kill one!
According to myth however, _should_ you encounter one, you can either deploy a "Buffy" :-), or use one of the following methods: http://en.wikipedia.org/wiki/Vampire#Destroying_and_avoiding_vampires
(the most-often cited method is a stake through the heart)