There was a thread around here recently about having temp HPs stack and 'bleed off' at a given rate, that would be a good addition to this.
I think that was mine, but it's since fallen off the boards. Here it is again:
Any temporary hit points you gain go away at the rate of 1 per minute, starting the round after they are gained. All temporary hit point stack using this rule; if you already have some from one source and gain more from another, they simply add to the total, and you continue to lose 1/minute. You can have up to 150% of your maximum hit point total; any bonus hit points gained beyond this are lost.
For example: Herne the 9th level cleric casts aid on himself. He gains 1d8+9 hp (we'll say 15). Five minutes later, his friend Bob the 9th-level bard uses his music to inspire greatness. Herne has 10 bonus hit points now, but he gains another 2d10 from the bardic music (we'll say 8), for a total of 18. Assuming he doesn't gain any more bonus hit points, he'll lose all the bonus points he has now in 18 minutes.
Hit points gained from energy/ability drain (undead, or the energy drain spell) are gained as healing, up to your maximum hit points; any bonus hit points gained beyond that point are applied as temporary hit points and follow the same rule as noted above.
So, for example, a wraith drains 6 points of Con from a fighter, gaining 30 hp. The wraith is wounded slightly in the fight - it lost 15 hp - so the first 15 are applied as healing, and the next 15 are applied as bonus hit points, which go away at the rate of 1/minute.
If you follow this line of logic, though, something weird happens when describing the damage type. The spell can't be said to generate negative energy for its damage, because it's not generating energy and you can't reasonably say that the energy is still negative when it affects the caster. The spell doesn't generate positive energy in its attack, because positive energy almost never causes damage unless you're actually in the Positive Energy Plane. The spell sort of... creates a sinkhole in the target's life energy.
Actually, negative energy makes sense - all level/ability-draining undead gain hit points from the levels/ability points they drain, and they use negative energy. Also, as mentioned, giving VT an energy type serves to balance it out admirably. It's a Necromancy spell, so having it use negative energy is not a huge logical jump.
But I would suggest removing the 'undead drain you' clause, Necromancers are suppose to be the masters of undead and magic to deal with undead, I would hope they would built their spells so as not to backfire on them if they were forced into combat with undead. Simply having it fail to work on undead is painful enough (the loss of a 3rd level spell and a successful attack roll).
YMMV, but I think if you're stupid enough to use a negative energy spell against an undead, it should backlash on you. No necromancer would be that dumb - they'd know better. Granted, there are a few cases where the undead doesn't LOOK undead, but that would be a nasty surprise that should be used sparingly.
I rather agree with the idea of constructs taking half damage, though. BUT, if this spell is designed to heal the caster, I think d6s would work just fine instead of d8s - that's a huge boost right there.