On this page of the Assassin's Creed Valhalla Siege of Paris guide you will find the exact location of the vault marked on the map from the Hidden Justice side quest.
Hidden Justice quest
You will begin this mission by reaching the northern part of this region. There you will come across soldiers who are fighting with the locals.
After killing the soldiers, you will be able to collect the reward.
The reward is up to your choice. You can get:
- Map and silver.
Regardless of which option you chose, the quest will end.
Map - vault
The map leads to the Murmiliacum vault, located in the northern part of Amenois. There, you will find a chest with 1500 Silver inside it. Apart from that, there's also some smaller loot.