We have a Google Apps for the Enterprise, with the Postini Archiving feature built in.
What is the difference between that and the Google Vault?
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Google Vault is a step on the way of integrating Postini-like services more nicely into Google Apps. That is, Google will probably at some point discontinue Postini-as-we-know-it, when all the relevant features have been reimplemented in Google Apps.
Thus, Google is transitioning Postini accounts to the respective Google features, including Google Vault. Your domain administrator will receive notices from Google with details sometime in Q1 2013.
So my advice would be to not delete your Postini account until you receive the transition notice, as Google will move your account to Google Vault for you, and your Postini subscription will be cancelled (from 2):
Starting in Q1 2013 and prior to your next renewal, you will receive a Termination Notice for your Postini agreement.