I am convinced that it has something to do with the area where you play the "Eye of a Gatelord" mission. If you go right at the very beginning of the raid to it you will see level 30 enemies. If you keep following the lasers in the sky to the gatelord spawning platform you have that giant portal. The lasers can maybe be turned to hit the spot next to the portal and activate it. Classic laser puzzle. Maybe through that portal is where we can find the chest. Or maybe the portal in the jumping puzzle leads there later on and the chest will be outside of the Vault.