Hello Graeme
One quick check, make sure that in the users IE, Tools, Internet Options, Security, Local Intranet, Sites, Advanced you see the proper Vault Alias listed.
This setting is what allows the end user to access their vaulted items via the shortcut in Outlook, without having to log in each session.
If the vault server alias is there, then the issue is something else.
You can try a couple of things, first removing the end users Hidden Message. The instructions in the following article will walk you through these steps. http://support.veritas.com/docs/287572
Once you have removed the hidden message, close Outlook and syncronize the users mailbox using the Syncronization Tab in the advanced page of the Archiving Task in the Vault Admin Console.
Have her open Outlook and test accessing archived items.
If this step didn't work, the second one that has worked for me was to create a new local profile. There may well be something wrong in her local profile. Rename current profile to old and test a new profile if that doesn't fix it either, rename her old profile back so that she will still have all her old settings.
Judy