From the official forums:
As infestation spread across the Solar System, Orokin Ships unable to make the jump to the Void locked down Vault rooms to preserve the valuable artifacts within. A number of these Orokin Vaults have been located in the Orokin Derelicts.
Are you worthy of the hidden treasures within these Vaults? New Corrupted Mods can be discovered here. It will take diligence and cooperation with your fellow Tenno.
To guarantee access to these Orokin Ships, you and your fellow Tenno will need to carry one of each of the four different Corrupted Orokin Vault Keys -- known as Dragon Keys -- into the Derelicts. Blueprints await you in the Market.
Once created, equip your Dragon Key as you would any other mission gear. Because these keys are so powerful, you can only equip one at a time. Be cautious. Each key has debilitating effects on Warframes.
When you find an Orokin Vault, only the CORRESPONDING Key will open the door. Team up with fellow Tenno, each with a different key, to guarantee your entry into the Vaults.
It won't be easy, Tenno, the rewards will not come without cost. Corrupted Mods offer great benefits but also bring negative effects and will need to be applied wisely.
Prepare by gathering your fellow Tenno to work together to ensure your success. More information will be provided today.
- The Lotus
EDIT: This is not an event. The Orokin Vaults will be a permanent addition to Warframe.
Get hype, Tenno!