I think part of the hoax is vault 74a even existing as canon. It was a test vault that was canceled in Fallout New Vegas that is inaccessible.
Although, shining light on it, it might be an easter egg that points to a new Fallout game that was placed in a secret place in the PC version to hint at vault 74a being of importance.
This quote from the fallout wiki:
"Vault 74 is one of the Vault-Tec vaults. The vault was not mentioned in any game and appears only in the game files of Fallout: New Vegas.
It was originally made by Bethesda Softworks developers for the official Fallout 3 G.E.C.K. tutorials. Despite not actually appearing in the game file of Fallout 3, it strangely appears as cut content in the game files of Fallout: New Vegas, although is only accessible through the use of the console commands."
...
"The vault comes with a simple quest called Exploring Vault 74. You start the quest by simply entering the vault (by the use of console commands) and you complete the quest by taking the Vault 74a overseer's log:
Transcript
Data Log #10.44.78
The Vault has been breached.
There was not enough power to level Five!
God have mercy on our souls."
Could mean absolutely nothing but it is a little strange that it appeared in Fallout New Vegas.