Bite of the Vampire by Terry Spear

Jasmine has the mission as a rogue vampire assassin to chase down a rogue in Wales. When she meets Stasio, she thinks he could provide a cover for her as her pseudo-boyfriend while she eliminates the rogue. But another assassin, who keeps stealing her missions, thwarts her. The next thing she knows, he claims Stasio is a rogue vampire! No way!

Stasio is a Welsh prince escaping the Dallas League of Vampires—with his three prince friends, and a witch, who his friend had turned and is now one of them—to return to their native Wales where they were also considered rogues. They’d tried to overthrow their League centuries earlier, lost their castles, and had fled. Surely, no one would ever think they would return. But then everything is turned upside down and Jasmine is stuck running as a rogue with Stasio and his friends.
But that’s not the end of it. When they arrive in Scotland to flee the peril, a group of Scottish vampires want to overthrow their own League and they want the princes to help them. Do they do it and risk it all? Will Jasmine leave them and find her own way? If they help them fight the vampire elders and lose like they did with their own League centuries ago, will they survive this time? These are the questions they have to answer before they get themselves even more over their heads than they already are and lose them this time.