In Canon, it was both the home of Count Dooku, and for a while the base of Darth Vader. In SWRP, it was considered for a long time the capital of the Sith Empire, mainly controlled by the Assembly of Darkness, but also containing other Orders including at various points Sith Legends, Dark Lords of the Sith, Order of Rowe and Dark Ancients, as well as its own set of Sith Troopers. Created by RR Almodovar (aka Darth Kestrel), Vjun was a vast system of walkways, buildings and towers. Originally containing the iconic Bast Castle, it was later reworked several times.
Funding for the sim was originally provided by Warpdone Vale, and this was later taken over by Desirae Carter. The sim was a key member of Galactic Unity and later "gave birth" to a sister sim initially called Vjun2, and which eventually became Korriban, also owned by Desirae Carter.
The sim was closed in the last quarter of 2008 after having been around for almost one year. The Sith who were based there moved to Korriban and Vjun's physical sim was reworked into non-SWRP sim.