The Orokin Vault is a special room found in missions on Deimos that is opened with Dragon keys. The vault door signifies the key required to open it (Chosen randomly from the 4 available keys). These keys are found in the Orokin Lab of your Dojo (If researched). A player can hold all four keys, but each key has a debuff.
Bleeding Key: -75% Health.
Decaying Key: -75% Shields.
Extinguished Key: -75% Damage.
Hobbled Key: -50% Movement Speed.
The player that retrieved the mod will get an additional (random) debuff of the 4 (which can stack with debuffs of the same type).
It is recommended to run a Deimos Capture mission in pursuit of said Orokin Vault. The door may look like this. After opening the vault and grabbing the artifact, all enemies spawned beyond that point will be Corrupted.
The Vault will not spawn in defense and assassination missions.
The Vault is a source of many useful mods such as, Blind Rage, Fleeting Expertise, Transient Fortitude, Overextended, Narrow Minded, and Heavy Caliber. You can view all of the mods here