Mercedes McNab is actually the third actress on the show from Buffy the Vampire Slayer (1997). The second being Amber Benson in season two's Bloodlust (2006) and the first being Julie Benz in season one's Faith (2006).
Mercedes McNab also played a vampire on the series Angel (1999); the spinoff of another show she was on, Buffy the Vampire Slayer (1997).