For being a psi-vampire, you have to be born one. Vampires are people whose bodies can't produce enough energy so they need other energies. You may also call them as parasites. So to meet their energy needs they steal others energy. Psi-vampires are energy suckers and have many psychic abilities which makes them totally opposite to Sanguinarius, the blood suckers.
As I said earlier, to be a psi-vamp, you have to be born as one. True that later you can later learn their abilities if you aren't born one. But that wouldn't make you a psi-vamp, you will just possess their abilities.
I hope i helped you.
If you need any further help, contact me, I myself am I a psi-vampire.