"You cannot install the Veritas Enterprise Vault Outlook Add-In by launching this MSI file directly. You must launch setup.exe. For more information, contact your administrator."

There are three options to resolve this issue.
Note: Pushing out the MSI must be done via Active Directory - if it is done with a script run on the Client machine, it will fail for the same reason, as it expects the setup.exe file to be present and used.
When a non-administrator client user on a workstation with UAC (User Account Control) turned on attempts to install the Enterprise Vault Outlook Add-In, it returns the following message: