Vanna was a Troglyte miner and leader of the revolutionary group known as the Disrupters on the planet Ardana. She participated centrally in bringing about substantial social change to the class structure of this misgoverned planet.
When the USS Enterprise arrived at Ardana to acquire zenite in order to stop a plague on Merak II, Vanna interrupted the delivery in order to kidnap Captain Kirk and so bring attention to her people's plight. Troglytes were being treated like slave laborers by the ruling class of the cloud city of Stratos, who evolved from the same original genetic stock as the Troglytes.
Plasus, the administrator of Ardana, believed that the Troglytes were inferior beings, but Kirk and his first officer, Spock, discovered that zenite gas from the planet's mines was temporarily retarding Troglyte mental capacity. Those who served in Stratos, as Vanna had done in Plasus' own house, were removed from the retardation effects.
Kirk finally held Vanna and Plasus within a sealed mine to stage a demonstration of the gas's harmful effects. After understanding the gas's effects, Plasus grudgingly promised to meet Vanna's demands and help her people. (TOS: "The Cloud Minders")