Before joining Karadin, she was a guerilla-fighter of the Scoia'tael until her unit got decimated, leaving her the only survivor of this incident. Out of a lack of options, she joined Karadin's assassins.
Vienne can be encountered in the Seven Cats Inn where she is getting drunk when Geralt of Rivia and Lambert enter the tavern. She can provide useful information for the quest Following the Thread, such as mentioning 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 that he is rumoured to have changed his ways. This can help later to choose whether Karadin should be spared or not.
When Vienne has finished her story, Geralt of Rivia can either advise Lambert to "do what he thinks is right" or plead Lambert to spare Vienne. If the second option is chosen, Lambert agrees to Geralt's suggestion and leaves the place peacefully alongside Geralt. If the first option is chosen, however, Vienne gets enraged over what she feels is just Geralt's hypocrisy and calls other guests of the Seven Cats Inn to her aid. A short fight ensues in which Vienne is killed.