@controversial-me If you didn't find the series
@Winterspring was talking about, then it's Karin.
Would like to add a few of my own: I think you'd know of White Blood; it's really nice, though the romance is not the main focus. There's Fortune Arterial, and Kyuuketsuki no Uruwashiki Kekkon features one female vampire coupling, iirc. You'd find a lot of female vampires in harem series like In Bura! or Jitsu wa Watashi wa, but I assume you're not looking for that. Hmmm, other than that, Bite Into Me may or may not be a vampire manga, and Dance in the Vampire Bund is pretty well-regarded, though I haven't read it personally.