Is this a bug?
Valen herself seems to be immune to level drain, which makes sense--she's a vampire, after all, and doesn't have any life-force (in the ususal sense) to drain. But she seems perfectly capable of draining the life from other undead, including other vamps.
It seems to me that all undead should be immune to Valen's level drain, plus probably any other creatures that aren't organically alive:
-Demons
-Planetars/Devas/Solars
-Golems
-Elementals
-I'm sure I'm forgetting a few
Maybe grant a few key bosses (Irenicus, Melissan, many Bhaalspawn) immunity to level drain as well? And tweak a few priest AIs to put negative plane protection in their repertoire? (Pre-Valen, it's an almost useless spell for an opponent to cast; now it makes a *lot* of sense.)
Maybe also move a few of the powerful anti-undead weapons (Runehammer, Daystar, Azuredge) so that some of them are in the hands of opponents, instead of chests.
This might help rebalance Valen without taking away some of her fundamental...er...vampiricism.