Card Text:
Mana Vault does not untap during your untap phase.
At the end of your upkeep, if Mana Vault is tapped, it deals 1 damage to you.
o4: Untap Mana Vault at end of upkeep. Use this ability only during your upkeep.
ocT: Add three colorless mana to your mana pool. Play this ability as a mana source.