Pranic energy, also known as life energy, is the energy needed to live. It is naturally produced by humans, and is in great abundance amongst us. Trees, flowers, your pet dog, they all possess it. You yourself possess pranic energy. Where it stems from is unsure, though many say possibly the nervous system. How? Beats me.
So how does this tie into Psychic Vampirism? Well, as mentioned before on the site, psi-vamps are unable to produce enough pranic energy for their daily needs (this includes using their empathic abilities). So a psi-vamp will feed off of a human who has pranic energy in abundance. This craving for energy can be very addictive. It's thought that the majority of psi-vamps have a psychological dependence on pranic energy. If a psi-vamp were to quit "cold turkey", he/she would most likely go through withdrawal. I say it to be a psychological dependence and not a physical one because I know of so many psychic vampires that did indeed stop feeding for good.
Pranic energy is what we all need to survive and live healthy lives. It's necessity is undeniable.