Versions

• Minecraft: 1.12
• Forge: 14.23.5.2847
• Vampirism: 1.6.2

Barons simply don't spawn in the vampire biome.

Reproduce Steps

Additional Information

The spawnrate of the barons is low, but it should work fine. (couldn't reproduce it)
Maybe there is a mod that is modifying something, but to find this out we need the log files.

In 1.12 this is confirmed to work.
They might only spawn during night time though