When you first enter the area from Serenity’s Waste (the very first time), you will see a huge structure right in front of you. You’ll need to get on top of this structure and onto the southern side, where you’ll see a bunch of ridges. The symbol will be on one of those ridges.
On the western side of the map is a huge tower, which has jump pads underneath it that allow you to reach a platform in the middle. Find a pillar that extends out from the tower on the west side of it, which should be close to the gate you passed through when you first got the moon buggy. In fact, use the top of that very same gate to reach the rocks that are raised nearby, allowing you to jump and "discover" the vault symbol on the side of the pillar.
This one can be gotten whenever you exit Deadlift’s place, as when you do, follow the map’s boundary to the east until you happen upon what looks like a piece of a ship sticking out of the wall. Right on this will be the vault symbol.