Way to unlock: Meet with the messenger.
The quest-giver: Messenger (M4,2)
This quest becomes available during your stay in Val Royeaux,
i.e. the location that you visit during one of the main quests of the game. Explore the central yard in Val Royeaux. You need to locate the messenger (M4,2) and talk to him. From the messenger, you receive an invitation to Vivienne de Fer's estate.
You can now leave Val Royeaux. Wait for the world map to load and pick the newly-unlocked point shown in the above screenshot. It is the Ghislain Estate.
After you reach the estate, you will not be able to explore it, because immediately, you will see a longer cutscene of the meeting with Vivienne. During the conversation, you need to decide, whether you want Vivienne to murder the Marquis, who slandered the Inquisition in public, or let him live. However, you need to take the most important decision towards the end of the meeting - you need to decide whether you want to offer Vivienne to join the party or let her go. Personally, I recommend the former.
Rewards for completing the quest:
- New Ally: Vivienne
- Experience (few)
- 80 Influence points
- 1 Power point