From where you first entered Triton Flats, via Moxxi’s Back Door, head straight west to find a small building that’s on the edge of the cliff. This is the same building you need to get flyers from in the Recruitment Drive optional mission. The symbol will be on the wall of the back of the building, on ground level.
Located in the Shimmer Island area, near the middle of the map, you will find a bigger building you can head inside, which looks like a garage (you enter here during Intelligences of an Artificial Persuasion to get the Stingray ). Head south from there and you’ll find a small path that leads to higher ground, but don’t take it. Instead, look to the east to spot a small building with two stories. Jump onto the outside of the second level to find the symbol on the outside wall, near the cliff.
In the northwest section of the map, you’ll find a large building that is a train station. If you go around to the northeast side of the building, you’ll find the vault symbol on it on ground level.