He would, but he didn't show up right after Vail left (he was timing his visits randomly and happened to get lucky) so Vail would be close enough by this time to hear her even if her voice is muffled. Also, Caleb is a Hygrace, so he isn't completely brutal like Spencer or other vampires. He acts more like Vail does by letting his pheromones do some of the work to make her relax and numbing her skin before he bites. Basically, he's slower.