So me and my friends played a bit tonight and were lucky enough to drop 3 vault key, and were able to use 2.
No screen, sorry about that, but in case you didn't know, keys drops from special cargo bot. They look similar but have 2, maybe more, glowing red hexes. When you break it, a key will drop, whatever the color of the box.
The first time we were pretty late in the game and no vaults were in the zone, but later, we got it near the train station at the start of the match, and got to open the nearby vault. We did that in almost 2 games in a row, too.
The loot inside is quite insane : the first time we got golden helmet, backpack and armor, with purple armor and backpack, and other purple stuff, and the second time it was golden helmet and shield with more purple items. We sadly lost the 2 times, but we won the first one, without the vault, so that's nice.