Rather than humans, the trap was meant for wild animals like bears. If a human were to get caught in it, even if he was wearing greaves, the trap would have fractured the bone. However, a vampire was completely different from a normal human in every aspect.
Although the trap was snapped firmly around her ankles, rather than a fracture, the vampire didn’t even seem to be in pain. In fact, she didn’t even seem to consider it an injury.