Close to the fast travel station is a bandit camp, where you should spot a tall tower-like structure that is on a hill. The vault symbol will be on north side of that tower, near the base.
In the eastern part of the map will be a building, with yellow trim on it, on top of a very tall cliff. West of it is a jump pad that you can use to reach that area. Head to the eastern side of that building and you should see the vault symbol on the wall facing the cliffside.
You need to be doing the Treasures of ECHO Madre optional mission or have already done it in order to get this symbol. If you don't remember where you went to reach Rabid Adam's hideout, it's the isolated section on your map in the northwest corner of the area. As soon as you get into the hideout, you'll go down some stairs. Go underneath the stairs and look up on them to spot the symbol.