Vienne is a former member of Jad Karadin's gang and an elven sharpshooter, being able to hit a sparrow from a hundred paces. Other members of the gang were Hammond, Lund and Selyse.
She is one of the people involved in the death of the witcher Aiden of the School of the Cat.
Before joining Karadin, she was a guerilla-fighter of the Scoia'tael. After her unit got decimated she joined Karadin's assassins.
Vienne can be encountered in Novigrad in the Seven Cats Inn, where she is getting drunk when Geralt and Lambert enter the tavern. She can provide useful information for the quest Following the Thread, such as the names of her former colleagues and where to find them.
Furthermore, she claims that Jad Karadin has cut all ties with his former employees and he is rumoured to have changed his ways. She recalls that she perhaps had a hand in Aiden's death, but not the details.
When Vienne has finished her story, Geralt can either advise Lambert to "do what he thinks is right" or suggest Lambert spares Vienne. If the second option is chosen, Lambert agrees to Geralt's suggestion and the two leave the place peacefully. If the first option is chosen, however, Vienne gets enraged over what she feels is Geralt's hypocrisy and calls other elven patrons of the Seven Cats Inn to her aid. A short fight ensues in which Vienne is killed.