The Vault of Secrets contains a text field and a mischievous padlock button below it, referred to as the Keymaster, whose true name is Glubfub. Pressing the button will cycle through a series of responses and clear any input in the text field. Entering certain phrases will unlock Icon Kit rewards and secret coins, as well as any related achievements. Hints in the form of green phrases are provided every so often to help guess the codes.

I just copy and pasted from the wiki

Robtop - Vault of Secrets