When a webcomic updates but you don't want to make a huge deal out of it, what you do

[Menarker]

Quoted: "Slaying a Vampire: Reducing a vampire’s hit points to 0 or lower incapacitates it but doesn’t always destroy it (see the note on fast healing). However, certain attacks can slay vampires. Exposing any vampire to direct sunlight disorients it: It can take only a single move action or attack action and is destroyed utterly in the next round if it cannot escape. Similarly, immersing a vampire in running water robs it of one-third of its hit points each round until it is destroyed at the end of the third round of immersion. Driving a wooden stake through a vampire’s heart instantly slays the monster. However, it returns to life if the stake is removed, unless the body is destroyed. A popular tactic is to cut off the creature’s head and fill its mouth with holy wafers (or their equivalent). "

I assume you're referring to the Roy/Durkula fight that is ongoing as of this post. The previous vampire in story that snuffed it (Malack) was killed via dispelling his protection from sunlight while in the desert sun. In this winter-mountain setting, sunlight isn't readily available, but there ARE two viable sources...
A: Vaarsuvius is capable of casting the spell Sunbeam, which would pretty much do the same thing. He/She might not have it prepared right this day, but if they can keep him unconscious, V can ready it.
B: If Sunna, that High Cleric of the Sun God, could be persuaded that a defeated cleric means that their vote no longer holds any sway, then he would be a more ready source.
C: A few pages back, as we saw Belkar falling down the mountain, we can see a river below. A few rounds of being submersed there would do the trick as well.

This is all assuming that positive energy that brings it below the killing threshold doesn't do the job on its own (and they are in a cleric-heavy surrounding).