Currently, negative damage spells are just useless, because Deathward spell can block it completely.
So, you can just ignore those threats when you have deathward.
Make deathward to give some damage reduction against negative spells. Like 30% reduction stacking with deathblock reduction gear.
This one will also open up new viable build on player side :
Negative Nuking Cleric or Pale Master, maybe Favored Soul(But, I think light/alignment based nuking is better-fitting for FVS flavor)
And, we need some ED that support Negative DPS, there is no ED that supports Negative DPS.
Battle against drow priestess, vampire will be real threat, not boring fight.