When Roan and Celis attacked the Bone Vulture for the first time, they gave Lukay and Rean enough time to move somewhere else. They hadn't truly disappeared; they had simply concealed themselves using Rean's Light Bending Skill.
To be fair, the Vulture should have paid more attention. If it had focused on its natural perception of cultivators instead of relying primarily on its eyes, it might have noticed Rean and Lukay moving toward the forest below. Unfortunately, it had no intelligence to speak of, so expecting it to make such a judgment was asking too much. Besides, how many times had it even seen cultivators in its life? One could probably count it on their hands.
