Follow the path as you normally would until you reach one of the elevators with a jump pad next to it, which should be the second one you encounter. If you look on the western side of the map, you should see an area that you will reach with said jump pad, which leads you to the Z8N-TP optional mission. Go on the opposite side of the structure and look towards the ground level to spot the symbol.
Throughout the zone are elevators that move upwards, with a barrier that will crush you to death at the very top of their journey. However, there is a single one that does not have this barrier, which will lead you to a secret area that is themed after a particular Italian plumber.
Inside this secret area will be a lot of enemies, so concentrate on them before looking for the vault symbol. Once it's clear, look up on the western side to spot a ledge, which is where you want to go. Use the floating blocks to reach it, where you'll find the symbol on the wall.