There is really no official recommendation from Veritas for Enterprise Vault client add-in deployment. But you can find information from Microsoft. See: http://msdn.microsoft.com/en-us/library/ms955915.aspx
However, you can reference: How to deploy Enterprise Vault Outlook Add-in.msi on multiple machines by using Group Policy http://www.veritas.com/docs/000081526 & Deployment of the Enterprise Vault Outlook Add-in via an automated deployment method may require End User interaction. http://www.veritas.com/docs/000037815
Also, an example might be to run elevated:
MSIEXEC /i "C:Veritas Enterprise Vault Outlook Add-in-en.msi" /qn /L*v c:client_installLog.txt
One thing to note that with Windows if UAC is enabled, then you cannot use the MSI. You have to install with an EXE. But if you disable UAC on users' workstations the install should be pretty straight forward (as above).
Other methods to elevate the MSI would be to package in an EXE that is a self-extractor http://support.microsoft.com/kb/197147 or create self-elevating scripts http://blogs.technet.com/b/elevationpowertoys/archive/2010/06/20/creating-a-self-elevating-script.aspx.