I wanted to illustrate that Vincent is a vampire who has a hard time stopping his desires. Scenes like this, where he things to drain Natalia, help to contribute to that character flaw.
From Inside the Grey House:
Vincent’s hands sank into Natalia’s hair, curling into her flesh, loving the feel and smell of her. He closed his eyes as the scent of her blood brought his body to life. He growled hungrily into her hair, trying to control himself. He couldn’t take her blood; he wouldn’t be able to stop. Hands were pulling her away and he didn’t protest.
Mierka held Natalia back as Joseph presented a human to Vincent. The human fell to his knees, dagger in hand and offered himself over to Vincent, who drank the warm blood down as Natalia watched, speechless. When he was done with the first, he stood and beckoned a second.
“What happened?” Natalia’s voice was shaking with wonder and worry.
Mierka answered as Joseph and Vincent both drank from their blood donors. “Edwin knew we were coming and knew we would try and save some of the children. It’s a game we play. He takes as many children as he can, knowing we have a chance to free most of them. This time was no different. Except that once the children were safe, we ran into some demons. They attacked us and nearly killed us, leaving us for the Morning Star to finish. Anthony was able to contact one of his men to come get us. We were badly drained.”