The Virtual Reality simulator, or VR simulator, is where the Vaul research improvements to their aerospace forces.
Despite having alloys comparable to the Protoss, the Vaul had to make due with lesser quality materials in order to avoid detection from the other three races. Because of this, their forces would be too weak to handle the Protoss in terms of raw power. To compensate for this, Simulators test the effectiveness of new aerospace technologies in both virtual and physical simulations before shifting into mass production.
Thanks to highly advanced Khaydarin crystal technology, these buildings can develop psionic weaponry that can augment the firepower of small fighters to equal that of heavy frigates and Battlecruisers. Using classified data copied by Mavericks, the Vaul can develop countermeasures capable of inflicting critical damage to heavily armored capital ships.