The Vulcan Ambassador to United Earth was the official representative of the Confederacy of Vulcan to United Earth.
From the formation of United Earth in 2130 to 2155, the Vulcan Ambassador to United Earth was Soval. At this time, the Vulcan Ambassador was often considered to be the "power behind the throne" for the United Earth government, since the word of Vulcan carried great sway with the UE government leaders. (ENT episodes: "Broken Bow", "The Forge")
In the alternate reality created following Nero's destruction of the USS Kelvin, Sarek served as Vulcan Ambassador to United Earth in the 2230s and 2240s even after the formation of the United Federation of Planets. (TOS movie: Star Trek)