All vampires have low level telepathy, for most of them, it's just part of their heightened senses. They mentally probe humans and other vampires alike. The more advanced ones can read and manipulate minds, but for most, sensing the blood in the room and projecting terror. Darla is hacking that. She has put him in a State where he can still read her mind, be he is left so weak that her thoughts are manipulating him, whereas normally, he would be trying to shoot terror directly into her subconscious.