The SimTek 5000 was part of the SimTek brand of virtual reality simulators, with one being located in Vault 13.
The SimTek 5000 was a VR simulation of the outside world after a nuclear holocaust in preparation for leaving the Vault for the Wasteland, specifically set in 2161 and 2287.
The SimTek 5000 is mentioned only in the Vault Dweller's Survival Guide, as well as the Fallout 4 Vault Dweller's Survival Guide.
Behind the scenes
The Tranquility Loungers were originally supposed to be new versions of the SimTek 5000s called the Simtek 6000s. However, all mentions of SimTek were removed before the release of Fallout 3.
Loading screen showing off what appears to be the SimTek simulation headset.