current mood: beset by vampires
"Psychic" vampire implies that the vampire in question has psychic powers, not that the vampire feeds psychically. It's the most common term used by the layman, but considered inaccurate by those who consider themselves to be vampires. Their preferred term is "psi-vampire." "Pyschic" refers to how the feeding takes place, while "psi" is what they feed on--psionic energy, also often called prana, chi, or etheric energy.
Sometimes, psi-vampires are fairly ordinary people who are suffering from some sort of illness, which has caused a loss of natural etheric energy, and so they begin to draw upon the energy of those around them to sustain them through the illness.
The symptoms of this include weakness, lethargy, frequent illnesses, and depression.
Since it's been scientifically proven that there are physiological differences between how an extrovert and an introvert react to social situations, all of this isn't really that surprising. An extrovert, by definition, gains energy from being around other people, while an introvert creates his own energy and is drained through social interaction. A psi-vampire could just be some extreme from of extrovert, incapable of sustaining herself energetically she must seek out those who create energy on their own to survive.
Now, this energetic extroversion doesn't have to manifest itself through perky behavior and social butterflyism. It simply means that in order to gain energy this psi-vampire must seek out creators of energy. Other vampires won't be good enough, because it isn't that being around other people creates energy for them, as it would for a normal extrovert. Instead, they literally drain energy, and so can only take it from renewable sources, lest they quickly starve.