Time Vault | Magic: The Gathering
Formats Time Vault is Legal in
Official Oracle Text for Time Vault
Time Vault doesn't untap during your untap step.
If you would begin your turn while Time Vault is tapped, you may skip that turn instead. If you do, untap Time Vault.
: Take an extra turn after this one.
Official Rulings for Time Vault
10/1/2008 : The choice of whether or not to skip a turn is made as that turn would begin, and only if Time Vault is tapped at that time. If you choose to skip your turn, Time Vault untaps before anything else happens in the next turn.
10/1/2008 : This wording means that you no longer end up skipping multiple turns if you find a way to activate the ability multiple times.