Minecraft Dungeons

The vault is a secret area of the Stronghold level. You can access this level immediately after downloading the DLC, however you need to collect 6 Ender Pearls from base game levels in order to progress to the other 2 DLC levels, and you'll need them for this achievement as well. The Ender Pearls are marked on the map and they're found in fairly obvious new paths in each level that lead to miniboss enemy that drops the pearl - you can quit the level once you have it and it will be saved.

Once you have them, you need to go to the Stronghold level and place them in the marked portal in the center of the map. You can either travel to the new DLC area and come back to Stronghold, or back out of the prompt and keep exploring. Now you need to explore the level thoroughly through newly openable doors and press 6 secret switches. Here are screenshots and map locations of each; I am not sure if the level is randomized so they may look slightly different to you.

Switch #1:



Switch #2:



Switch #3:



Switch #4:



Switch #5:



Switch #6:



Once you have hit all 6 switches, go to the area behind the marked portal and you will find a newly opened door, head on through. I ran around a bit in this new area and the achievement randomly popped but you can also explore to find a few items.

Door Location:



Full image album:
https://imgur.com/a/18Yw1Mz

The Stronghold Vault can only be opened after finding all 6 Eye of Enders and activating the End Portal in the Stronghold level. After doing that, load up the Stronghold level and search the side paths to find five hidden buttons. Activating all of them will open a secret door in the center of the room near the End Portal. Enter it to find treasure and your achievement