From Protector of the Grey House:
Natalia reached out and caressed his closed fist. He opened his eyes when he heard her gasp. “What did you do?”
Surprised by her question, he looked down at his now open hand. There were four half circles of crimson on his palm. Blood pooled in the shallow of his palm, and ran down the sides of his hand. He hadn’t even felt it. Vincent opened his other fist and saw a similar scene. He frowned into Natalia’s eyes. Her eyes were clouded and dreamy, as if drugged. He almost asked what had transpired between her and Diana when Natalia brought his hand to her mouth and licked his hand clean. Once the tiny pool was gone, she sucked on his hand. Vincent growled, closed his wounds, and gave her his other palm to clean.
When his hands were clean, Vincent turned Natalia around and pressed her back into his chest. Reading his intent, she brushed her mostly wet hair away from her neck and tipped her head to the side. He growled into her skin and bit her shoulder as gently as he could. She tensed with the pain but relaxed into him when he sucked her blood.