On a new savefile, get the vault ending without unlocking any of the ZUG doors. You can only use the backdoor found in the village.
Do not use the "cheat code", do not use Developer mode, and do not directly edit the savefile via Notepad.
1. Hop over to the secret area, then warpbomb everything in there - including yourself. If you're speedrunning, you can optimize this by leaving out one black gem.
2. Unlock door A with the master key and head to the village.
3. Read the inventory book from the L tower.
4. Use up your rainbow gem and buy a warp gun. Inventory it.
5. Use up two black gems and buy a blue bird. Inventory it.
6. Go in the cave between the Cubes shop and the L tower. Solve the Amazing Button Puzzle and inventory the gems, do not use them up.
7. Go back to the save room. Use your newly bought warp gun to bring down a grey gem from the upper corner.
8. Take one gem out of your inventory. Inventory a warp bomb and go to the village. You should have one blue bird, one warp gun, one warp bomb, and one gem in your inventory right now.
9. Using your blue bird and your inventory, fly (https://nifflas.lpchip.nl/index.php?topic=6844.msg72376#msg72376) to the famous backdoor, found near the upper-left corner of the map. Throw the yellow bird out of the way. Enter the backdoor.
10. Deposit all the gems on the floor and the gem you have in your inventory into the vault collector.
11. Release the silver trophy box. Throw your warp gun against the ceiling near the box, so that its "trigger" is pressed down and it warps the box.
12. Release the golden trophy box. Stand directly below it, then take out your warp bomb and ignite it. Make sure you get the box (and yourself)!
13. Now you're back in the save room. Get the silver trophy out of its box, then put it in the yellow box. Put a gem in the yellow box as well. Put the yellow box in your inventory, as well as the 2 other required gems (you need one blue, one green, one gray, and one black. one has already been deposited and one is in your yellow box) and a blue bird. Right now you should have a yellow box, two gems and the blue bird in your inventory.
14. Using your blue bird, fly to the backdoor again. Toss the bird. Enter the backdoor.
15. Drop the bronze trophy box. Then deposit all the gems & trophies into the vault collector for an "easy" win.
Walkthrough video coming never because I can't get it right :/