Sounds a lot like the side backstory of this, it’s a BL though, but the story of the girl is she is sold off to a vampire master by her father king as a treaty between them.
http://www.mangago.zone/read-manga/anaerobic_love_prisoner_of_the_deep_sea/
Or this maybe, but no vampires, just demons and fairies, but the FL is a swordswoman and the guy is a smith with powers.
http://www.mangago.zone/read-manga/seiken_no_katanakaji/mf/v01/c001/1/
It’s neither of them but thank you for linking these stories!
I’m searching for a webcomic.
I only remember the beginning:
There was a (noble?) girl who did swordsmanship. Then she learned she was in an arranged marriage with a dude who was the best swordman of the country. I think the dude had black hair. (And maybe he was also not human but a vampire? I’m not sure about this part) and then we had a flashback about the girl’s dad beating the dude in a duel and then asking him to marry his daughter and this is why the dude wants to marry the girl (because he promised the dad) but the girl doesn’t want it at all. And I think the girl had blond hair.
Voila that’s all I read at the time. The story was on Tapas but then got deleted.
Thanks in advance!