You need to defeat 100 vampires in Barovia. While you are progressing towards completing the campaign for that other achievement, you might get this achievement along the way. If you don't, you want to head down to the SE of the map in the Ruins where Vampire mobs will spawn. You can run around farming your kills there, both in the day and in the night.
Another good place to farm is heading to 'The Taken!' Heroic Encounter when it comes up, also in the Ruins, which has a good amount of vampire creatures in it if you get there early.
You can see your progress if you go to Journal --> Achievements --> Slayer --> Vampire Slayer