There is the equivalent of typically four to six liters of blood in the human body. For a creature of human proportions to survive on a strictly blood diet they would need at a minimum two liters of blood a day. If one does the math that means there is the requirement for at a minimum fourteen liters of blood in a seven day period. It is likely that a creature surviving on a blood diet and gaining the psychological and emotional thrill from the kill would in fact feel a real need to kill and not just feed on his prey.

If this vampire then controlled him or herself they would perhaps kill only once or twice a week. Bearing on the side of caution and saying that a vampire only killed on person a week, then that is still four people a month, and four times twelve months a year leaves a long and very evident trail of bodies behind them. This would naturally cause quite a stir and have the authorities calling out all the stops to catch the serial killer on the loose. These are some biological and inescapable facts of what a real vampire running loose would face. That means in order to get the very minimum of food they require they would have spend quite a few hours of the day searching for an adequate victim and using their powers of seduction and superhuman strength to acquire them. This then would mean that a real vampire would operate almost like an animal in that nearly all their waking hours and energy would be used for hunting. So in review there would be no suave sophisticated vampire running around spouting philosophical platitudes and enhancing their cultural outlook.

They would be using Sex and force and anything else to feed. They would concentrate on feeding constantly and the use of Sex would only be a very minor tool. They would have a desire for cock or pussy only insofar as it gave them access to a meal and nothing else. The reality of vampires then would be a much darker and more different one than is popularly imagined.