As a vampire, it is best to know yourself well enough to keep your blood donors safe.
From Inside the Grey House:
Vincent was alone for some time before Joseph knocked on the door. He didn’t wait for an answer but came into the room, leading two human males. The humans sat in the chairs facing Vincent, not saying a word. None were necessary. One, the elder, placed a small dagger and a roll of gauze on Vincent’s desk.
“Would it be safer for you to take from me or them, sir?”
The men didn’t appear to notice the question. They were well trained. There were many times Vincent’s humans had been in a room with Vincent and Joseph. Sometimes Vincent fed from them; sometimes he fed from Joseph who then fed from the humans. It was a sign of good faith that Vincent knew when he was too hungry to feed safely from a human.
“You made a wise choice in bringing men. I’ll be fine drinking from them.”