What is a vampire group called?

A vampire group is called a coven.

It is actually called a 'Brood'.

Brood - A group of vampires gathered around a leader (usually their sire).

Sire - The parent-creator of a vampire, used both as the female and male form.

A brood may (over a long time!) become a 'clan' by virtue of lineage, bloodline.

Clan - A group of vampires who share certain mystic and physical characteristics.

A 'coven' is only correctly used in terms of witches... --noun: an assembly of witches,esp. a group of thirteen. However since more people know more about witches than vampires and since 'coven' also means 'gathering/assembly', the Word coven are sometimes (wrongly) used for a group of vampires too.