The rebels must be wondering why I bother to help them. Well, they have no idea what power is in that vault...
Shortly before the battle, Legion spies in the Empire discovered intelligence concerning a new piece of Imperial technology codenamed the Impervious Device, a machine capable of rendering an object invulnerable to all harm by resonating particles. They also informed 8311 that prototypes and data concerning this device was hidden in the Imperial vaults on Coruscant. 8311 was planning an attack on the vault when the Alliance contacted him, asking them to help raid the vaults for information on Imperial troop movements in exchange for a large sum of money. 8311 agreed, knowing that the rebels would provide a good distraction. He met with rebel leader Mon Mothma and the two agreed on the battle plan.
Once the battle plan was arranged, Legion and rebel troops met up in the lower levels of Coruscant near the vault. After the combined force was assembled, the attack began.
Oh, it's probably just a bunch of terrorists. Get rid of them.
—An Imperial officer
The two forces attacked from the bottom of the building where the vault was located, rather than the main entrance, where the most Imperial forces were concentrated. Imperial forces within the vault building were taken by surprise and overwhelmed, but quickly the Imperial commanders there worked out what was happening and dispatched reinforcements. Rebel MAL vehicles, brought on the black market, held off Imperial AT-ATs outside the building as a distraction.
The Legion elite soldiers, the Legion Defilers, led the Legion force through the building to the vault where the device prototypes were held. Making short work of the stormtroopers guarding the vault entrance, they blew it open with thermal detonators and quickly took the Impervious Device prototypes, as well as various documents and blueprints in the vault. In order not to make the rebels suspicious, some of the Legion troops assisted the rebel freedom fighters in bringing down various Imperial walkers.
Breaking into the Imperial vaults, the rebels gained some of the information they were looking for, but when they heard that Darth Vader was heading to the building to investigate, they retreated. The Legion forces covered their retreat by blasting apart parts of the buildings with bombs, before being picked up by Legion CR20 transports. Fleeing the building, the rebels scattered around the surrounding city, before escaping in freighters.
Commander, you allowed a rabble of mercenaries and rebel terrorists to break into our vaults and steal valauble technology. A punishment is in order, I'm sure you'll agree.
—Darth Vader before Force Choking an Imperial commander
Arriving at the scene, Vader punished the Imperial officers stationed there before informing the Emperor. The Holonet covered up the incident by claiming that the explosions were the result of a reactor failure. However, nobody knew of the Legion's involement; many assumed that the Legion troops seen there were merely mercenaries hired by the rebels.
Meanwhile, Legion technicians studied their prizes, and soon discovered, much to 8311's annoyance, that the devices were merely empty casings with some rudimentary circuitry. However, they had not left completely empty-handed; they had discovered intelligence of an Imperial research base on Mygetto that was focusing on perfecting the device. Immediately, 8311 ordered a raid on this base, which would lead to the Raid on Mygeeto.