Start out at Peepot's place, which is where you got The Empty Billabong optional mission. If you look at your map, there's a little piece of land sticking out from the main portion slightly northwest from where Peepot's house is located. Go to that spot and look at the ground to find the vault symbol.
At the southwestern end of the lava river that runs through the center of the map is a small island that is marked on your map. Jump to it and you'll find a loot chest, as well as the vault symbol on the ground behind the chest.
Head to the comms station and go through it normally until you enter the first area with oxygen. As soon as you go back outside, jump onto one of the objects nearby and get onto the rooftop of the building you just exited. The vault symbol should be on the wall right there.