There are three vault symbols hidden in Concordia, all on the rooftops, so go to the fast travel station and use the jump pad there to easily access the rooftops.
Right by where you spawn with the fast travel station is Janey Springs’ shop, which has a big sign. The symbol will be on the ground right behind the sign.
This one is on the little piece of rooftop that is right above where you entered from Serenity’s Waste the first time. Basically, from where you used the switch to shut off the electric barrier in Systems Jammed, you can jump to the rooftop and find the vault symbol on the ground part of it.
On the back of the tower thing that’s above The Meriff’s office. This is the spot where the turrets spawned in Systems Jammed. You want to go where the highest turret was located and around to the backside.