The Vegan Tyranny was an interstellar political entity controlled by the native Vegan species of Vega IX in the Alpha Quadrant. The Tyranny once controlled a vast area of space, however by the time of the Federation the Vegans had gone extinct, and subsequently little was known about them; some speculated that they may have been a cybernetic race. (ST reference: The Worlds of the Federation)
When the USS Enterprise was accidentally sent back in time to 1969, Montgomery Scott reminded James T. Kirk that the Vegan Tyranny wholly dominated space outside the local group of stars, and therefore it was unsafe for the Enterprise to go there. (TOS - Star Trek 2 novelization: Tomorrow is Yesterday)
By the 23rd century the Vegans had gone extinct and Vega IX itself had been colonised by both Delta III and the Federation. One of the relics of the Tyranny still present on the planet was the Vegan choriomeningitis disease, a synthetic microorganism created by the Vegans. The disease was not detected until it had been carried to several other Federation worlds, resulting in a widespread outbreak. Fortunately a vaccine was quickly developed. (ST reference: The Worlds of the Federation)
- The Vegan Tyranny first appeared outside the Star Trek universe in James Blish's Cities in Flight series of novels. Blish established them in the Star Trek universe in his novelization of "Tomorrow is Yesterday".