Through a Glass, Darkly
The events surrounding how Vulcan officially became a part of the empire following first contact with Humans in 2063 have been lost to time. Some say the Vulcans joined the empire believing they could manipulate Humans from behind the scenes, while still others suggested that the Vulcans joined feeling a sense of weakness and a need for self-preservation. Having fought a devastating war against the Romulan Star Empire between 1939 and 2041, it could be the latter case which prompted Vulcans to offer themselves as vassals of the empire, rather than full partners. Whatever the case, Vulcan and its people would play a key role in helping to expand the borders of the Terran Empire.
T'Pau, who had been a minor diplomatic official at the time of first contact, would eventually become Vulcan's head of state. It was through her actions and her ability to manipulate one Emperor after another that allowed Vulcan to be both a vassal world within the empire and yet maintain some of its autonomy. Initially T'Pau lured emperors into her bedchamber through her own charms, but then later used a cadre of seductresses in the Imperial Palace to sway and influence the empire's officials. Her agents were equally adept at wielding psionic powers and had access to a huge collection of ancient Vulcan poisons which alluded medical detection.
During the glory days of the Terran Empire, T'Pau, the reclusive mistress of Vulcan lived in a citadel on the Plain of Tai-La. Its roofs had gleaming liquid ludugial gold held into place by force fields, musk-scented corridors, secret passageways, spy holes and a torture chamber. (Through a Glass, Darkly)
Age of the Empress
The fortunes of Vulcan would change radically in the 2150s as a rebellion had broken out against the empire. When the Empress Hoshi Sato was taken prisoner by the rebels while en route to Deneva, she enlisted the help of her former shipmate, T'Pol, in gaining her freedom. Although T'Pol had joined in the rebellion against the empire, she switched sides after uncovering a plot to murder her by fellow rebel T'Pau. For her assistance to the Empress, T'Pol was named the Supreme Regent of Vulcan. Moreover, Vulcans and their world were elevated to a status equal to that of Humans and Earth within the empire. (ENT novel: Age of the Empress)