Decoy vaults

Overview
Decoy vaults make up 89% of Vault-Ts actual size. Decoy vaults are usually large portions of hallways, usually very run-down.

1800's
Decoy vaults were not used for the way they are used currently, The decoy vault was intended to be break-rooms for miners, Decoy vaults would of been built in mines so the miners would have easy entry.

1902
When the Vault-Corp started subjecting people to toture, They would lock people in the mines if they started slacking. These portions of mines did not get gassed, however people would die of Hypothermia because of the low temp. This is when the Decoy vaults were used as punishments.

1988
When Vault-T became as large as europe, many parts of the vault were not needed, and with the ongoing research of RD, they used the decoy vaults to test the disease. The guard in the picture is most likely guarding the room where RD was being tested. The locked mine punishment was still used.

Present
Decoy vaults right now cover an area 2x the size of europe, 3D printers made randomly generated hallways for miles and miles, it is possible that the decoy vaults are as large as the entire northen hemisphere. Many creatures live here because of extensive Bio-Research.