Many organisations have shared mailboxes for functions like HR, Admin, Finance, IT and so on. In some organisations which deploy Enterprise Vault, they enable the users for Exchange mailbox archiving, but don't enable those shared mailboxes. The question then is what happens with those shared mailboxes in the Outlook profile?
There are two parts to the answer...
Firstly the Enterprise Vault toolbar doesn't change its 'shape' .. This means that the buttons which are shown are determined by the policy on your mailbox, if particular folders, or other mailboxes have differing policies the buttons don't get enabled or disabled on the fly. So in this situation where the user may have a full selection of buttons like 'Store in Vault', Restore and so on in their own mailbox, when they go in to the Shared Mailbox, they still have those available.
Secondly since they have these buttons available, end-users might try to use them. The good news is that Enterprise Vault doesn't just 'allow' the operation to take place just because the button is available. Checks are made, and in this case, the shared mailbox is not enabled for archiving, so an error is shown to the end user. [Of course this could be improved, because right now the error is that a 'server error occurred'.]
If you have Shared Mailboxes, and you see shortcuts to archived items in there, the only way that they could have gotten there is if an end-user dragged and dropped them from their mailbox to the Shared Mailbox. This is okay to do.. but.. when other users who also have access to the Shared Mailbox try to open that shortcut they will get an access denied-type of message, because they don't have access to the first users archive.
Using Shared Mailboxes with Enterprise Vault requires a little bit of user education. Once the possible actions, and results are understood they can carry on being a good way to implement a business process.