Head to the central part of the map and find the Air Dome Generator there, which is on the ground level. Find the "H-shaped" structure close by and stand on it, looking to the east, where you’ll spot a Dahl sign. To the right of that Dahl sign is a container with horizontal stripes, which is where you’ll find the symbol (it’s on the north side, close to the ground).
Head to the Pumping Station in the southwest corner of the map and go to the west end, where you’ll find a jump pad. Use it to get to the next level, where you’ll find another jump pad, so hop onto that one and get on top of the walkway above the Air Dome Generator. There will be a tower to the east and west of you, so concentrate on the western one.
If you look on the north side of the western tower, you should see ledge running out from it. Jump onto that ledge and look at the tower to find the vault symbol.
This one is located in the southeast corner of the map, which is somewhere you’ll be if you do the Nothing is Never an Option optional mission. You should see a lone building here, with the vault symbol being on the south side of it.
Head to the northwest compound and go inside, where you’ll see a covered tunnel straight ahead. You will want to head to the far back of the compound, on the same path as the covered tunnel on your map. When you’re there, the vault symbol is on a wall at the north end.