Yeah, as I said. I can get the 'they're undead and must be destroyed according to your oath" idea... except the only reason the quest is happening is the vampire I've been travelling with, and not destroying. Which has never caused the slightest problem with my oath.
So I guess we've got a classic paradox of the heap going. Letting one vampire live after showing they're more than a mindless monster and can control their hunger is okay, while 7000 is an oath violation. So would 2 be okay? 6999? What number of vampires requires me to destroy them, vs it being okay to be friends?
As I say, it's not game breaking- I just paid the thousand gold and told the oathbreaker knight to go away- but does feel like there's some real inconsistency in what the oath allows.