Showing Beyond the Pale by Savannah Russe

The government knows that Daphne Urban is a vampire, and they have an ultimatum: spy for them, or be killed. The choice is easy. She can speak 13 languages, has a genius IQ, and has escaped detection for nearly five hundred years-making her perfect for Team Darkwing. Her first mission is to get close to Bonaventure, a shady arms dealer with an unexpected gift for seduction. But when Darius, a darkly sexy vampire slayer, begins chasing her, Darius is torn between desire and duty. For his lithe, young prey is also his ultimate temptation.