Enterprise Vault has a very cool way of handling what happens when an item is manually archived, from an end-user operation, when they have Vault Cache already configured.  The way it proceeds is as follows:-

 

* User clicks on 'Store in Vault'

* Enterprise Vault Outlook Add-in marks the item as pendingarchive.

* The Add-in sends a HTTP request to the EV server to process the archiving of the item

* The Add-in stores the item in the appropriate .db file in the content cache

 

This way the item doesn't need to be downloaded during the next synchronisation run since it has already been added to Vault Cache.  It saves a round trip to the server for each time this operation is performed.  It's quite cool.