The Mystery Vault is new to Mafia Wars. It is a randomly appearing pop up that players are seeing from time to time that gives them the opportunity to find some limited loot, Reward Points or experience points.
To make use of the Mystery Vault you must click on “Email Me The Code!” You will have to give Zynga your email address of course.
After you have provided your email you will get a new email message from Mafia Wars or Zynga with a link in it that you will have to click. When you click the link you will be returned to your Mafia Wars page where you will be presented with a new message, which you must click in order to open the vault.
You can find random limited loot items, 1000 Reward Points, or 250 XP. To be totally honest you are most likely to find the experience points since giving away 1000 reward points would be counter productive to a game with the sale of reward points as its revenue model.
Good luck to everyone and may you find what you seek in the Mystery Vault.