One of the things I like about WP/VP is that I can set up psychics who use their VP to fuel powers. Any thoughts about replacing that mechanic?
I remember that Torg has a Backlash mechanic, but despite my habit of posting about it a lot here, I haven't played it much to use it in practice. But basically, you took damage if you rolled to low on your casting roll (and the DC for casting a spell was often lower then the DC for avoid backlash). You don't need vitality points for this, especially because you probably want to eventually invoke the feel of the possibility to kill yourself when overextending yourself.
One of the things you need to be aware of when you use Drain/Backlash:
Your hit points / vitality points suddenly become way more important. Give up the idea of spellcasters having less hit points then non-spellcasters. If you don't like that, you need to give them a kind of extra pool of points that they can use for that very purpose - and stacks reasonable well with the rest of the system.
Alternatively, you could allow spending "willpower" points or something like that. They would work identical to wound points, but are based on a different statistics. Still, more hit points remain important. You either end up with a spell point system, or all characters having the same hit points and toughness in combat situations. (But spellcasters lose their hit points at a faster rate,since they have two sources to take damage from - enemy attacks and their own attacks.)
If you use vitality points that recover fast, you also change the balance of powerful utility spells - vitality point cos basically is meaningless, unless it kills you.
Torgs backlash damage works a little different still - you roll your skill total, and the degree of difference to the backlash determines damage. A low toughnes spellcaster still tends to drop unconscious faster, but the way damage is accrued depends stronger on his mental statistics and skill, as one would want from such a system. Shock points are important, but the attrbiute or ability that sets the damage result value is still way more important. The moment the attribute to determine the amount of shock you can take is decoupled from the attribute to determine how much damage you take in the first place, the latter is more important.
But translating that elegantly to D&D/d20 systems is not so easy. You might want multiple success levels on a task like casting a spell or using a psychic power...