It's been a while since Vanessa and Susan had a break from roaming Washington and ducking from every vampire faction roaming the area. Vanessa finally has a break from looking for the whereabouts of her daughter. She needs a break. Looking for her daughter, and only that, will drive her mad.
Susan, before she was turned, one of the few familiar things in Vanessa's life. At least the life she can remember, anyway. Vanessa's forgotten how bewitching Susan's lips were, how enchanting Susan's eyes were, how warm her embrace was.
For the moment, Susan wants to forget about this new life that seemed to generate in front of her eyes after she awoke in the hospital. And it doesn't seem like anybody needs Vanessa at the moment. So she's going to make love to Susan until the two of them were needed to dispense of some vampire threat yet again.