If it makes spells 25% more effective against the undead, then how does it improve armor enchantments and potions? Those aren't spells.
Enchantments definitely ARE spells. You don't cast them, but they're considered as such. Potions...well. The game essentially considers all active effects a "spell", even if it really shouldn't be. This extends to potions, food (I think), and probably even diseases/poisons. The last two vampires don't have to worry about, anyway.