The Remnant Tiller
This quest is revealed after completing From the Dust. Follow the navpoint given to a dome-sealed vault. There's a lootable box in the large shipping container, so crack that open, then descend vaultward.
The moment you touch down, you'll see two Observer generators activate. Shoot them down, then use the console on the right (the only interactable console in the room), to summon an Observer of your own.
Scan the spinny core on the high platform, then scan the console near the door to get your new pet Observer to open it (Zap, unfortunately, cannot do the same).
Through the door you'll find a number of shield bubbles infested by Assemblers. Clear them out.
Break right (west) and see the only shield-buff bubble in the area. Pass through it and turn right into a small room.
Clear out the Remnant here, then look north to find another Observer generating console.
Summon the Observer. The rules here are that this observer will only work on the westmost set of rooms here. If you scan an Observer console, it will come unlock it for you, even if it's not that close, but it cannot pass through closed doors. If killed in combat, a new one automatically appears by the original generator. Give this one a test drive by scanning the console down the western slope of this room.
The console opens up two alcove, on north and one south (this is a repeated structure throughout the level). Each alcove has console, and each console is marked by a certain number of lights. Activate the 'One' console to the north, then the 'Two' Console to the south. Nothing happens yet, and that's fine. Jog up the slop, but not all the way back to the main area. Instead, bear south, and enter a little airlock-type room with a shield-bubble and a console.
You'll see a lot of this structure too. Activating the console (by hand) will switch the bubble, close the door behind you, and open the door in front of you. Make sure your pet Observer is with you, then cycle the lock. The next room has a relatively large force of Remnant, but nothing you haven't seen before.
Clear them, and activate the Observer console to the west, positioned the same way as it was in the previous room. Again, two alcoves will open, with two more consoles. Activate 'Three' in the south Alcove and 'Four' in the north.
This open up the airlock to the southmost room in this part of the complex. Cycle the lock for another warm reception.
Nullifiers and lesser beasts. Bust 'em up, and jog west. You don't even need an Observer console here, as the diamond alcoves are already open- a container and spinny core to the south, a tunnel with a glyph to the north
Note that, most likely to some last minute revisions, there are more glyphs in this Vault than you actually need. I suspect the final activation was originally much harder. There's also a large container in the north-east alcove. Grab it, then cycle the airlock to return one room north.
From here, a Remnant bridge will take you to a center island, then across to the east side. Before you cross, look left for a spinny core in an alcove, then assault the usual welcoming committee on the east bank.
Destory them, head through the airlock south, and destroy those jokers too. There are some open diamond alcoves along the eastern wall- the north has a small container, the south has an ammo box. The north alcove on the west side of the room, however, has this wing's Observer generator.
Take your new buddy through the airock north, and unlock the alcoves there. The north one has a container, and the south has a spinny core.
Airlock north again, to join up with the larger room. There is an Observer console to the east, though, just like in the rooms south. Dispose of light resistance there and activate the console to reveal- a hostile Observer. Sweet.
Kill that sucker, loot the container it was guarding, and scan the right & left lower walls for two extraneous glyphs.
Now airlock all the way south for the vault's big fight- An elite destroyer flanked by nearly a dozen assemblers. As ever, try to clear the herd so you can focus on the big guy, but be careful not to get caught out in the open. In particular, don't be afraid to retreat all the way back to the door to secure some breathing room when you need it.
When the coast is finally clear, activate the Remnant Console for the Tiller, which can be solved like so.
A new bridge also opens up, bridging the gap between here and northern main room, but we're not quite ready to leave. Use your Observer to open the alcoves to the east (if your Observer died in the fighting, airlock back to the generator on the east side, then airlock south again). The north alcove has a small container, the south alcove has a container, a glyph, and two scannable data patterns on the wall.
Now you can take the central bridge back to the surface. If you want, there is a small container on the rock formations in the north east of the main room you can snag before you go.
Return to the surface for a Viability reward, and new source of fuel for the Initiative