I played it for the very first time, Saturday night. I get to the treasure room and open the side chests first and then open the main chest. I got Truth out of the main chest. Anyway, I'm playing on Sunday and go to move a key from my Titan to the Vault, thinking I only have four in there. It won't let me because I still have five. I played POE with my Warlock and had taken the five keys out of the vault for POE and thought I'd only put 4 back. Obviously, I was wrong. Before anyone suggests it. No, I only ever acquired 6 keys. Three came from scavaging chests and three came from completing the story with three characters. Does the lvl 28 main chest not use a key?