An ethical psychic vampire is basically a kind of psychic healer.
Psychic vampires siphon energy from others because they feel a need to do so. To behave ethically means to have a greater degree of conscious control over your behavior, and you can't take away energy from others ethically if you do it because of a need you feel.
There are many situations where taking away psychic energy is the right thing to do. For example, a common type of headache is caused by mental confusion which allows the mind to overload with stressful psychic energies. In a way, psychic energies can be cancerous as well. Balance cannot be establishes if there are surplus energies in the system - they have to be drained first.
Generally, psychic vampires just take and take without any regard. They respond to a show of force - not to an appeal to ethics. A psychic vampire who is doing his or her best to behave ethically is on the way to become a psychic healer.
What differentiates a psychic vampire from a healer is not the amount of energy they take (a competent healer can drain massive amounts of psychic energy), but what they do with it afterwards. A psychic vampire has no conscious use for that energy and simply uses it to feed and bloat the ego, whereas a psychic healer lets the energy flow out and establishes a greater balance.
When to give and when to take? The flow you create determines the quality of your existence.