The wording and behaviour of Time Vault has changed over the course
  of the game.  Nowadays, it has a relatively simple wording:

  Time Vault  {2}  Artifact 
/ Time Vault comes into play tapped. 
/ If Time Vault would become untapped, instead choose one - untap Time
  Vault and you skip your next turn; or Time Vault remains tapped.
/ {T}: Take an extra turn after this one. 

  This wording uses a modal replacement (you choose which replacement to
  apply when applying the replacement) and forces the controller to use
  up a turn to have the untapping of the Time Vault actually succeed.
  This removes a few extremely abusable loopholes with the previous
  incarnations of the card, but copy effects and bounce effects can
  still make for a potent combo.

Some parts © 2009 Advameg, Inc.