Vincent is used to getting his way. That is not so with Natalia. Spoilers in this section. I think this scene shows us more of Vincent than even the vampire would be willing to show.
From Inside the Grey House:
Vincent brought his favorite human as close as he could, exposing his fangs as he dipped his head to her vulnerable neck. His lips parted slowly to the soft rhythmic pulsing of her vein. His teeth scraped her delicate olive skin, making her arch her neck. She was such a willing victim. His hand traced down her elongated neck as his head pulled back. Both Natalia and Charlie told him of her reaction to the werewolf attempting to change her against her will. She had thrown Charlie out of her life. Would she do the same in this situation?
Vincent continued to caress her neck, allowing his hand to follow the curve of her breasts down to her stomach. He believed she would leave. Not only leave but disappear. He had a great network of spies, but even they had problems following her when she wanted to be left alone. She would disappear, perhaps never to return. He embraced her again letting her touch burn away his intentions.
He wanted her to stay. If it meant letting her be human and having her risk her life fighting to help him gain control, so be it. He would talk to her about changing her if she was dying during a fight, but he would never change her without her permission. Vincent wrapped his cool limbs around Natalia’s warm body, inhaling her scent as he took a deep breath.
“With your permission Natalia, and not a second before.” He kissed the back of her head gently and fell asleep holding his favorite human.