The Vulcan diplomatic enclave in Sausalito, near San Francisco, which housed Ambassador Soval and his staff during the decades when Vulcan advisers oversaw Earth's progress into space.
The Vulcan compound is the residence and workplace of Vulcan's representatives on Earth in the twenty-second century. It stands in Sausalito, across the bay from Starfleet Headquarters, and is referred to throughout Enterprise.
Ambassador Soval and his aides are based there, and T'Pol lives in the compound before being posted to Enterprise in Broken Bow. For humans such as Archer it stands for the Vulcans' habit of advising, delaying and holding back technical help.
T'Pol's decision to stay with Enterprise rather than go back to duties there is a recurring thread of the early seasons. In The Expanse she resigns her commission with the High Command rather than be recalled.