Vector Sigma is the spheroid supercomputer located deep in the bowels of Cybertron which gave birth to the Transformer race. The existence of Vector Sigma was revealed when Megatron--furious at the Autobots' undisputed dominion of the roads--traveled to Cybertron to find Vector Sigma so that he could build his own team of road warriors, the Stunticons. Without the aid of Vector Sigma, the Stunticons were no more than lifeless husks which could only be guided by remote control.
Optimus Prime and a team of Autobots pursued Megatron to Cybertron but were too late to prevent Megatron from stealing the key to Vector Sigma from Alpha Trion which was necessary in order to activate the supercomputer. Before Megatron gained access to Vector Sigma, he and his Decepticon squad was confronted by a cadre of nigh-invincible guardian drones. Megatron cleverly used the key to command the guardians to attack the approaching Autobots, since they were programmed to protect and obey Vector Sigma, and the key was an integral part of said computer. The Autobots encountered the guardians and, despite a valiant attempt to push through, were denied entrance and forced to retreat into an old hangar. Running short on time, the Autobots resorted to a desperate ploy. They rebuilt some abandoned worker drones to fight the guardians in their stead. Wave after wave of worker drones proved no match for the indomitable guardians, but the last wave of worker drones walked aimlessly off an edge. The primitive guardians pursued their quarry off the edge and met a similar end.
During this time, Megatron had taken the Stunticons to Vector Sigma and had successfully activated the supercomputer, and Vector Sigma granted life and personalities to Megatron's new troops. Megatron took the key with him as he and his Decepticons returned to Earth. Disheartened upon learning this, the Autobots were disgruntled when they decided to fight fire with fire. Inside the hangar they had retreated to were old Cybertronian freighters which they modified into Earth aircraft, and thus, the Aerialbots were born. However, since Megatron had left with the key, the Autobots had no means to activate Vector Sigma. Alpha Trion, who had accompanied them, plugged himself into Vector Sigma and merged with the supercomputer in order to activate it to give life to Prime's new soldiers. Prime, too, departed from Cybertron with five new faces.
In a related note, Megatron discovered that the key to Vector Sigma had special properties on Earth. It could emit a beam of energy which transformed organic matter into metal. Gleefully, Megatron set out to turn Earth into a second Cybertron whereupon he was set upon by Prime. There, the Aerialbots and Stunticons clashed for the first time and established their long-standing rivalry. In the ensuing melee, the key to Vector Sigma was destroyed by the Aerialbot leader Silverbolt.