I finally got selected for the mystery vault - sorry for the delay in reporting this; I just never ended up getting asked for the bonus! Here is what it is and how it works:
When you log in, in what seems a random fashion, you have the chance to get this pop-up:
Note: You MUST have your email address submitted or you will not see this pop-up. You must give Mafia Wars permission to e-mail you or you will not be able to take advantage of this feature. You can add your e-mail at the "set-up" tab at the top of your Mafia Wars screen.
Once you have the pop-up and your e-mail address submitted, simply click "Email Me The Code". You will receive in email in a few moments.
Unlike the text says, you don't actually get a code. You get a link that you can click on in your inbox. Simply click on it, and you will log in and see an option right on the home page to "Open the Vault". Click that and get your reward:
So, the feature isn't too bad. It's not great, but it's not bad. What you'll get is 2 decent loot pieces, a random amount of experience, and a random amount of Reward Points. I seriously doubt more then 10 players will actually win 1000 reward points from this feature; it seems that the common range is 1-5. Still though, a reward point, a few decent items, and 100 XP for simply clicking a link is not a bad deal at all.