Just going to come right out and say it. I think that perhaps the single stupidest mechanic in the entirety of AD&D is Level Drain. The very premise is absurd as the attack and the consequence are completely and entirely un-related.
A vampire attack leaves you drained of life force within you and you lose experience levels? I'm sorry, I don't think that one was really thought out all the way and just got chumped to get the book published on time or something.
To lose experience levels, I could see it if the attack or damage was cognitive in effect. Something stole your memories or you got amnesia or dementia or something along those lines. Then yes, drain those experience levels and rightly so.
To lose life force though? Here's how I homebrew it anymore. Instead of losing an experience level, I remove Hit Points and 1 point of CON instead. I do use Hit Dice though. so if say a 6th level fighter were to take a life force draining attack, I will subtract on full HD of HP. Fighters roll a d10 for HP thus I remove 10 HP for each life force level drained. Add to that losing 1 point off of their Constitution. Life is going to be different for that PC forever after.
Sucks to be the victim in that attack but it makes WAY more sense than to lose XL The PC retains the XP level they have though. So, 6th level Fighter now has 10 less HP but is still 6th level because the attack didn't suck part of his brain out with the knowledge, learned skills and experience that makes him what he is. Losing a point of CON starts to reduce their ability to be resurrected or have Raise Dead done for them. It also kicks them in the HP bonus adjustment as well.
I want the life force drain to be scary. I want Players to think twice, maybe three times before tangling with something that could very well end it all for them. So if the 6th level fighter had a max potential of 60 HP but with some lousy dice rolls only had 48 HP to start with, losing one life force level automatically knocks them down to 38 HP. Bang! Just like that. Get another life force level drained? Lose another 10 HP.
It's an easy mechanic to keep track of, it's relevant to the attack and it makes those creatures causing it something to be carefully considered. Which is the whole point of having level/life force draining creatures to begin with.
Having said that, I always leave the door open to having HP re-gained by magical or other means as well as finding ways to get CON points back too. It won't be easy but it is possible. Those things are already built into the game for exactly those reasons it would seem.
Plus, the endeavoring to regain those HP and CON points can make some really nifty solo and side adventuring for those PC's as well.