How do medieval villagers regularly donate blood in a safe manner once a month? I’m pretty sure sterile venipuncture equipment wasn’t available and leeches would digest the blood.
I know bloodletting was practiced, but was it done on a given patient on a regular basis? Slashing the whole village’s arms once a month would lead to heavily scarred villagers and probably a number of infections, wouldn’t it?
Also, if I was that vampire, I’d probably agree to the deal while I had the stake against my chest, but then get the hell out of that village to find new digs on the first opportunity. The knowledge that the villagers knew where I “slept” would make it hard for me to feel secure.