Let me explain, yes there is psi all around us. But in people it is already in a high concetration and it is easier to tap into. Think of psi as air. Well there is air all around us but if your've got a breathing problem you need concentrated air to help you. So basically psi vamps are astral lung cancer suffers. wink
Ah yes, standard answer, I was waiting for this. Ok, you're right! The secondary problem to this whole 'psi stealing' thing is that you don't actually need psi to live, you really don't. You can remove all the psionic energy from your body and be completely fine. I've tried it, it works, and I'm still alive to type this.
People seem to think 'More psi equals more power! So I shall have all the psi in the world!', and this is not necessarily the case. Most psionic abilities (for instance scanning) take no gathering of psi at all, and the ones that do often take very little psi. For example; constructs are constantly being formed without you being actively aware of it (this is why we must filter often) which take very, very little psi to make.
You have to consider psi as something that just helps the process along, greases the wheels, it's the soul that does the real work.
As a sidenote, if you were to become proficient in creating psi, I'm sure you could create amounts of psi comparable to that of a human. It just takes some time and effort like anything else.