Virtual Reality

Virtual reality or VR simulators were a fairly new technology before the Great War and are known to have been in use only in the United States military and in at least one Vault-Tec Vault.

Interestingly, the extent to which the simulators, for lack of a better word, "simulate" reality is unknown- in some cases, being killed in the simulation results in the person being killed in real life. It is possible this is only an extreme case, (as it is a failsafe in Vault 112, and a method of unlocking an armory in the VSS Facility.) Since, as previously stated, these simulators are known to have no civilian or entertainment applications, the question remains whether they were part of a previous training simulation or other project, possibly involving the development of power armor. Again, due to the limited information available since the bombs fell, this may never be known.

Known VR simulators
There are two known VR simulators - Vault-Tec's Tranquility Lane simulation in Vault 112 (run by Stanislaus Braun as part of the Vault Experiment), and Virtual Strategic Solutions, Inc.'s Anchorage Reclamation simulation in the VSS Facility (now known as the Outcast Outpost).

The simulator at the VSS Facility is labeled "SIM-U-TEC Chamber 003", suggesting that at least 2 more simulators were built by VSS Inc. Whether these were functional is unknown. Possible is that there where simulator rooms behind the doors, you see them when you go down with the elevator.