Yeah. Durkon has figured out that while he can't directly lie about anything to his vamp bodynapper, he can choose which "true" information to reveal. This was shown when he let the vamp know that the keystone held by the head cleric was needed to unlock the door to the Thor temple, but didn't reveal that if the keystone wasn't given freely, it would disintegrate instead of unlocking said door. I'm assuming that was intended as foreshadowing for other bits of information he's giving the vamp that are incomplete, and that the missing bits will prove to be significant.
Which leads us to the likelihood that there's some other reason in addition to the one he revealed as to why he'd want the vamps in that hall. Don't know what that is, and I'm probably somewhere down around 5% on accurately predicting anything in this strip, so I'm not sure how valuable speculation will be. We can reasonably assume that it's not that someone else will be there instead of the owners (he said "a place where thar willnae be anyone ta kill right now"). So no convention of vampire killers who meet each Tuesday morning while the banquet owners are away or anything. It might be something about the hall itself that gives an edge to the order, or perhaps a disadvantage to the vamps. Maybe it's been outfitted with an automatic smoke clearing system, after the great kitchen fire of '08 or something, so if they turn into mist, they'll get sucked into a storage tank maybe? Or Tuesday doubles as cleaning day, with the cleaning staff being made up of dwarven made stone golems or something (so no one to "kill"), who will gleefully "clean up" any vermin in the hall (including vampires).
Or... It's something totally different. Yeah. My money is on that.