Psychic (Psi) vampires feed on people's qi, instead of blood.
Psi Vamps tend to be more readily able to practice psychic arts such as Clairvoyance and Astral Projection.
one way the feeding process has been described as for Psi's is sending an ethereal tentacle or leech to another's chackra or focal point of energy, and then drawing it in through there. thats direct feeding though another way is to go into a large crowd and drain from the energy just floating around, a little here a little there so no one notices.

Related Link: Human Energy Manipulators vs. Psychic Vampires