Defiance by Mysti Parker

Wren —vampire princess, collector of men—is on the run again. But she’s tired of being on the defense. The time has come to infiltrate the traitress queen’s inner circle and destroy her plans.
And then to overthrow the queen herself. But when Wren and her mates crash the monthly party, they reveal just how deep the queen’s corruption goes. Turns out, she has a depraved slave cult with the motto They who are truly served.
Aka T.W.A.T.S.
Despite the ridiculous acronym, Wren and her guys serve up some satisfying death and destruction, and it’s enough to get the queen’s attention.
Charles—a mercenary, a joker, and now a potential member of Wren’s harem—is used to keeping five steps ahead of everyone.
But Wren has him spinning ass over ankles. Not only is she gorgeous, but she’s ruthless about digging up her mother’s past.
Unfortunately, that involves his past, and his connection to the former queen’s death. Not to mention that joining Wren’s harem means becoming one of the good guys, and that sounds like an awful lot of commitment. If only it weren’t so damned fun…