In the vault bounty there are hidden stuff happening.

1) You get the "necessary" bonus stage after hitting the pillar, it leads up to a small parazon puzzle like in the zone tutorial, lots of crate got some mech parts in there

2) in the first big vault room there are 2 operator switches(can be hit by operator weapon), it opens wall doors and there are switchs in those closet(on the top not on the ground). You need to manipulate those to make the rings statue in the center of the room go full vertical, then you can jump in there to be warped in a secret room with a mech to kill and loot :)

Also nekros seems to work on the mechs. It's all I have for now :)