This vault symbol is located inside of Janey Springs' place, which has a downstairs and upstairs portion to it. The symbol is underneath the stairs leading to the downstairs area, behind a container.
You might spot this one while doing the Tales from Elpis optional mission, as it's in the same area as the first ECHO you have to get. If you've done that optional mission, then you remember the ship that the ECHO was hanging off of. Jump on top of the ship and you'll find the symbol on top of the fin part that's sticking out of the ship.
This one is on top of the building you went into to get the broken Oz Kit at the beginning of the game. The symbol is on the side of large part sticking out of the roof.