Indeed, I didn't fully understand the description of the spell, specifically "the unit gains all the properties of the undead," and immunity to unholy words is one of the properties of the undead. :) All that remains is to acquire 2 spells of level 5. And what else is so great about vampirism? It is quite effective, even if the hero doesn't have much magic skill. A stable 50% of the unit's health will be drained, plus an additional 2% for each spell power, which in any case guarantees more than vampires. :)
Bow down and hear our cry,
Our earthly rulers falter,
Our people drift and die,
The walls of gold entomb us,
The swords of scorn divide,
Take not thy thunder from us,
Take away our pride.
G.K. Chesterton: English Hymnal