Prisoners are a tool used in V Rising to replicate Blood Types and blood quality of an enemy mob. To accomplish this, you must first have Dominating Presence unlocked, then once you've found your desired enemy and have dominated them, you must take them to a Prison Cell. Once there, you will have the option to drain blood using an Empty Glass Bottle, which removes 30-60% their HP multiplied by their misery percentage and gives you a vial of their blood, or to feed your prisoner various Fish and Rats to reduce their misery and raise their HP.
Misery is the multiplier added to the lost HP when you feed on a prisoner. The equation for damage done to a prisoner when draining blood is:
(DB*PF)*(ML+100%) = DD
DB=Drain Blood %
ML=Misery Level %
PF=Prisoner Flooring % (100% for Basic Flooring, 75% for Prison Flooring)
DD= Damage Done %
Without Prison Flooring = 65% - 230% misery
With Prison Flooring = 120% - 340% misery
The Misery equation is a best guess based on the observations that were gathered. This might not reflect the actual calculation happening in the background of the game. Feel free to correct this information if you have further knowledge on this.