So I checked Lexicon and found out the following about vampires (link):

1. They are studied in DADA but not considered wizards.

2. The vampire Harry meets at Slughorn's party is described as pale and gaunt.

3. They are hated and feared by many.

4. There are vampire hunters.

5. Centuars oppose them.

6. Garlic is used against them.

But my question is, how would a wizard kill a vampire?

Would the vampires have joined You-Know-Who?