The hungry farmer perk is unlocked after maxing out the farming tab in the skills mod. The skill essentially auto feeds players until they reach max saturation. It does this by using food items in the inventory.

Vampires don't have a max saturation, and can constantly eat food. Because of this, any vampires that have the hungry farmer skill cannot have any food items on them without the skill immediately "eating it". An example of this is having a stack of bread on your hotbar will cause it to immediately get wiped.