Google Apps Vault eliminates all sorts of administrative headaches for Google Apps and Postini customers such as managing duplicate user lists, email aliases, and email distribution groups across the 2 systems.
Currently Google Vault is only available to new Google Apps customers but we expect Google to allow the migration of existing customers from Postini to Google Apps Vault soon.
By the way, Google Postini is still the best cloud spam and virus filter for Microsoft Exchange or other emails servers on the market. It will continue to be supported and enhanced for the long term as there is a huge install base of existing users. Since ~90% of all email is spam, Google Postini removes this heavy load from your internet data lines and on-premise Exchange server. When we switch customers from an on-premise firewall based spam filter to Postini or Google Apps they are often shocked by the dramatic drop in load on their internet data lines.