Vannis was a female Vorta and an assistant of Weyoun.
In 2376, Vannis was sent by a Founder to learn more about the Ascendant Raiq, who Odo had heard about from the Sen Ennis earlier that year. (DS9 - Worlds of Star Trek: Deep Space Nine novel: The Dominion: Olympus Descending)
In 2383, Vannis met with Benjamin Sisko when Sisko arrived on the Founder homeworld seeking clarification about Dominion involvement with the Typhon Pact. Vannis was adamant that she would not allow Sisko to meet with the Founders, but Sisko asked her to contact Odo and tell him he needed to speak to him urgently. (ST - Typhon Pact novel: Raise the Dawn)