Possible Glitch Edit
I believe that I may have encountered a glitch. Darion initiated combat with me, ran into the bathroom area and disappeared. I defeated every Kahn in the vault, but the quest remains unfinished because he's vanished. This prevents me from completing the deal with the Tandi, thus getting the good ending for both the Vault 15 Squatters and the NCR. Very upsetting.
I'm not sure whether or not it's only me or what I did to achieve this. I'm playing on the PC. The reason I'm bringing this up here is because it could be added to the bugs section.