Why would the vampire agree? If they need one person a month and can create thralls and vampire spawn at will then why should the interest of the peasant be considered at all?

If you want a medieval society that really is better off than a feudal one, it's easy. Just kill the lord. Nothing is really lost here. Just make them communist. The feudal hierarchy did not exist to benefit to peasants, it was an institutionalized form of robbery defended by a military class of warriors/knights and justified by religious authorities.

Making the lord a vampire actually makes the imbalance of power greater. Why should Stradh settle for less than a mortal man? He'd just as simply keep a few dozen humans as cattle, and pay for their sustenance through taxes!

If you look at feudal government as transactional (which it wasn't) then yes, obvious getting superior protection for a lower cost is better, but this could describe a relationship with any powerful creature you could think of so long as you decide they would agree. Dragon, Sphinx, Naga, Djinn, if you make the case that they're more cost effective than a human lord then there is literally no arguing with you because you are the DM.