To enter the First Vault, you must collect 10 user coins then go to options then click the lock in the upper-right corner. then type these in:

From 2.0

• Spooky - ShyGuy Icon
  
• Sparky- The Vaultkeeper's Secret Coin
  
• Ahead - New Wave
  
• Lenny - Lenny Icon
  
• GandalfPotter - white Trinagle Trail
  
• RoboTop - New robot
  
• NeverEnding - New UFO
  
• Mule - New Ship
  
• BlockBite - Crying Soul UFO

From 2.01

• '8','16','30,'32','46','84' (enter these consecutively) - Illuminati wave
  
• *enter user's Name* - Illuminati Icon

Source From Geometry dash wikia

To enter the Second Vault you must collect 50 diamonds
Diamonds are retriveable from daily levels, Quests and Chests

After unlocking the Vault enter these Codes:

Octocube - one eyed octocube
Seven - Finn form Adventure time
[insert # of stars you have] - New icon
The Challenge - allows you to play the vaultkeeper's level this costs 200 diamonds to play
Brainpower - Icon with it's Brain Sticking out form the top of his head
Glubfub (make sure to get the Special dialogue about it from the first vault or else this code won't work)
Video: https://www.youtube.com/watch?v=tudoQBmOa34 - Source Partition On YouTube
thechickenisonfire - Secondary colour (green)
gimmiethecolor (type it in the american Spelling or it won't work) - Primary colour (red)
cod3breaker (read below on how to solve this)

Memorize/ Write down your 6 numbers
Label them as
1 2 3 4 5 6.
THEN you do have to do:
No.2-No.1= A
No.3-No.2=B
No.4-No.3=C
No.5-No.4=D
No.6-No.5=E
The code is
ABCDE
Do not times them together
Note: THE SEQUENCE IS DIFFERENT EACH TIME! YOU MUST GET THE CODE YOURSELF!
Video: https://www.youtube.com/watch?v=01zISiu2XhI -Source Norcda Childa


Main Source : Geometry dash Forums

The Timekeepers Vault or the final Vault is hidden pretty well by RobTop

Step 1
To find it you must go to the Coming soon page on the official levels page, then click in the middle of the the Ground decorations. This is around where the Page number dots are...

Step 2
Then a door will appear. once it appears, the timekeeper will request a Special item. To get this item you need to go to the second Shop (scratch's shop). this requires a minimum of 5 keys and 500 diamonds to unlock.

Step 2A
If you don't know where to find his shop then click on the online levels page. Then click on the door in the bottom right-hand corner. go to the very last page and pull on the rope

Step 3
you must buy the special item from the shop this costs 1000 manna Orbs

Step 4
Go back to the TimeKeepers door and unlock it with the special item.

The codes

Riddles

• (1)The thunder comes before the lightning,
  And the lightning comes before the cloud,
  The rain dries all the land it touches,
  Wrapping the earth in a blood red shroud.
  What am I?
  
  
• (2)What Runs but never walks,
  Often murmers, never talks,
  has a bed but never sleeps,
  has a mouth but never eats?
  
  
• (3)shh,Quiet!
  Can you feel it colsing in?
  Shadows Creep where is begins,
  Speak it's name and it is Gone
  From every moment, Dusk till dawn.
  
  
• (4)I hide behind rocks and rills,
  Under the mountains and hills,
  i cannot be seen or felt,
  nor heard or smelt,
  The more i am the less you see,
  What am I?
  
  
• (5)The less you consume,
  The stronger i become,
  Wait too long...
  and Death will come

Riddle answers / Codes

• (1) Volcano - New wave
  
• (2) River - Colour (green)
  
• (3) Silence - New icon
  
• (4) Darkness - New icon
  
• (5) Hunger - New icon

Tutorial
https://www.youtube.com/watch?v=N72a-c0Ey6M

Source Norcda Childa

I'd like to thank the community for Finding these. You guys are truly amazing. And i apologize for the previous Guide.
don't be afraid to let me know how i could improve this guide and/or creating more guides in the future
Hope this Guide made up for it and did help most of you :) thank you