A vizier was a high-ranking advisor to a monarchy in some Eastern cultures. The Doctor encountered several viziers, both on and off Earth.
In 1289, the First Doctor encountered Kublai Khan's vizier at Khan's palace. (TV: Marco Polo)
The Fifth Doctor encountered the Sudanese vizier. (AUDIO: The Destroyer of Delights)
The Third Doctor encountered the Sclaponian Grand Vizier. (PROSE: Island of Death)
The Eighth Doctor encountered the glass Vizier of Fortalice. (PROSE: The Scarlet Empress)
Bernice Summerfield also encountered an individual who called himself the Vizier, whilst on Perfecton. (PROSE: Oh No It Isn't!)
While in 1626 Paris, Erimem told King Louis XIII that the Doctor was her vizier. (AUDIO: The Church and the Crown)