Thieves can steal from the vault, it is not 100% impenetrable. Vault-owners may seek to maintain the "illusion" it is by replacing the gold, but gold may not be there some days.
I have to think a bit more about this, but still it seems to me "gold not stolen from vault" is far more reliable than "human being being able to do work of type x" both at given time.
Understand that the Value staked is never actually redeemed except in default.
Issuers would redeem the voucher for some equivalent to "1 hour" job.
They may prefer to give gold, or ?
