To allow mods to e.g. add blood values without any code/API requirements

Send a inter mod message to Vampirism.
Use FMLInterModComms#sendMessage(String, String, NBTTagCompound) during pre-init or init to send a IMMessage.
Use key blood-value to send a NBTTagCompound containing a String id with the entity id (yourmodid:entity) and a Integer value containing the value.

Also supports no sundamage biomes now 08e110d
Just use key nosundamage-biome and the ResourceLocation id of your biome as value.