If the registry entry has been set to prevent the expansion of PST files on users' computers, then you need to perform the following actions in order for the Vault Cache feature to work.
You need to request the following Outlook hotfix from Microsoft and apply it (but note that the hotfix may already have been applied as part of a Microsoft Update):
To configure users' computers
Install the latest Enterprise Vault Outlook Add-Ins on users' computers.
Create the registry value PSTDisableGrowAllowAuthenticodeOverrides in the following location:
For Outlook 2003 - HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\11.0\Outlook\PST
For Outlook 2007 - HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\12.0\Outlook\PST
Be careful to use the path that is shown here.
The setting must be of type REG_DWORD, and have a value of 1.