Natalia challenges Vincent to a fight, to get her way. Here, she recites his rules to show she knows what she is doing:
From Inside the Grey House:
“Do you know what happens now?” He started to circle her.
Natalia held his gaze, turning as he did. She came across his challenge rules in one of the Slayer journals. Vincent didn’t like when his men tried to kill him. If one of his men had a problem with some rule Vincent established, or they couldn’t settle an argument, they could land one blow then stop and the challenge would be enacted. “We fight. If you win, you’ll drain my blood, if you so desire. If I win, I get to drink half your blood. Since I’m not a vampire and I don’t want to drink your blood, I have a proposition.”
“You’re in no position to propose anything, human.”
“I swept you off your feet fairly easily, vampire. Do you really think I can’t do that again?”