FUV light is absorbed by air below 180 nm. Purging a monochromator with a gas (most often, Nitrogen) will evacuate all the air contained inside, and will enable working down to 160 nm, even 140 nm if a degassing by baking is performed.
Vacuum is necessary for VUV spectroscopy if you need to work below about 120 nm; otherwise FUV light is absorbed. The level of vacuum can be either HV (High Vacuum, from 10-3 mbar to few 10-8 mbar) or UHV (Ultra High Vacuum, from 10- 9mbar to 10-10 mbar). These two levels of vacuum correspond to two different technologies: for the HV version, sealing is made by Viton gaskets, while copper gaskets are needed for the UHV version. Moreover, design and manufacturing processes for UHV systems need very specific attention. For example, in UHV, one has to use metallic bellows or specific feedthrough to transmit a movement from the outside to the inside of the chamber. This leads to needing completely different mechanical designs between HV systems and UHV systems, and makes the UHV solutions more expensive.
Most of the time, the choice between HV or UHV version is driven by the application, and by the setup in which the VUV system will be connected. A monochromator dedicated to be mounted on a synchrotron beamline or onto chambers where electrons are produced, are most often, the UHV version.