The answer to the first question is that Google Vault allows domain administrators to retain and search all incoming and outgoing emails from a Google Apps for Business domain. This is often required by businesses that decide to retain this information for legal purposes (in the event of a legal dispute this information can be requested).
Up until now, this job has been served by the use of 'Google Message Discovery' by Postini, but the benefit of Vault is that it is part of the Google Apps domain rather than a separate product. That's going to make it much easier to configure and use, always a good thing.
At this stage, Vault will only be available to Google Apps for Business customers, and not users of the education edition for which GMD will remain the recommended product.
The cost of Google Vault is expected to be equivalent to that of Google Apps itself (£3.30 / user / month) and as a Google Apps Authorized Reseller, I'm looking forward to this being part of the product offering. If you'd like to discuss the use of Google Vault with your Apps domain, please feel free to get get in touch.