Challenge [Hard] (Spoilers): Vault ending using 2 doors

Head to the secret room to the left of the starting location and teleport two teleport bombs and the master key, but nothing else, not even yourself. Then reset and do it again. The third and fourth time around, only teleport two bombs. The fifth time, teleport yourself while carrying a bomb. This'll give you two master keys and eight teleport bombs.

Unlock the Z and G doors, then head through the former with a teleport bomb. Throw it up the duct where the gold trophy is dropped just before detonation. If you time it right, this'll give you not only the decorative gold trophy in the wall, but also the reusable box containing the reusable three item box with the other gold trophy. It's still possible to do this if you only get the decorative gold trophy, but slightly more annoying.

Use another teleport bomb to retrieve the red gun. Next, drop a blue cube in the large gap to the right of the gold trophy room. Bring the red gun there as well and push it into the hallway below, then use the cube to get back out. Repeat the same process with two teleport bombs.

Next, go through the G door, push the button to drop the slippery reusable item box containing the silver trophy, and throw the teleport bomb into the silver trophy room. It's still too far away, so repeatedly throw a bunch of gems (and optionally the bronze trophy) up there to slowly push it closer. However, make sure one of the green gems is left behind. Be careful none of them fall into the valuable collector. You can block it using the box the bronze trophy comes in. Once the teleport bomb is close enough, repeatedly throw the red gun against the ceiling until it hits the teleport bomb. Be careful it doesn't shoot you or the remaining teleport bomb or green gem. Once that's done, use the other teleport bomb to bring the green gem back to the save room.

Use another teleport bomb to get the white and black gems from one of the crevices in the save room. If the previous teleport explosion in the trophy room didn't give you any blue gems, put a blue cube and the yellow bird from the three item box into the animal collector to get a blue gem. Then reset.

Now it's finally time to bring the gold and silver trophies as well as one gem of each color (except rainbow) to the valuable collector. If you got the reusable three item box earlier, you can put three of them in it. Otherwise, just transport them separately. Put another blue cube into the gap in the gold trophy room and push the things you need down to the hallway one by one. Then go through the G door and gather them in the collector along with the valuables lying around.

Go to the secret room left of the starting point. Use one of the bombs to send the other two bombs and the key to the stack (but don't teleport yourself). Then go into the menu and select Reset Puzzles. Return to the secret room and teleport yourself, the key, and the bombs to the save room.

Unlock the U door. Take one of the bombs through the door and use it to teleport the decorative silver trophy in the wall. Take another bomb through the U door. Use some blocks from the save room to push the bomb through the gap, so that they fall into the area below.

Now go back and unlock the G door. Pick up the bomb and throw it into the gap below the Z door (to the right of the buttons.) Use the gems by the collector to push it through to the other side, making sure you lead with a green gem. Then walk to the other side, push the button to release the gold trophy box, and activate the bomb to send yourself, the trophy box, and at least 1 green and 1 blue gem to the save room.

Use another bomb to teleport a white and black gem from the upper left or upper right corner of the save room. Reset. Now take one blue, one green, one white, and one black gem and the silver and gold trophies through the U door and push them through the gap using random blocks from the save room again. Go through the G door. Release the bronze trophy box. Place all of the gems and trophies into the collector to get the Vault ending.