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.
: Untap Mana Vault at end of upkeep. Use this ability only during your upkeep.
: Add three colorless mana to your mana pool. Play this ability as a mana source.
[1E]:R [2E]:R [2U]:R [3E]:R [4E]:R [5E]:R [ME4]:R [VMA]:R [MPS_KLD]:S