If you wanna get technical, Evil clerics (like most Vampires) are not necessarily restricted from the usage of any spells for various reasons. If they are, it's because they are Good-aligned (or some deity-specific restriction), which often means they're Positive Energy spells.
Divine Damage does not affect undead in any special way whatsoever, it's just another damage type. Specifically, the power of the Cleric's god, albeit in an unholy form rather than holy (which NWN does not emulate).
So, an Evil Vampire Cleric could cast Hammer of the Gods simply because it's a generic offensive spell with no real restriction.
The same priest could conceivably cast Searing Light as well since it's also a generic damage-type spell like Hammer. The only issue I would see from an IC perspective is using it on a non-undead target which would severely weaken the spell. Unfortunately, there are very few offensive Cleric spell that low-level (that I know of) besides the Inflict spells, and those are Touch spells instead of Ranged spells. Meaning, that semi-squishy cleric would have to get into melee intentionally to cast it. Which is kinna dumb since they have Warriors and Rogues for that job.
This is not a DM view but a personal view - that perhaps Vampires worship evil deities, da? And evil deities still provide divinity. Do sourcebooks say that divine damage is good-aligned only?
^--- Like he said, Even the evil ones can get divine powers. There are evil gods, undead ones, too.