Outlook 2010 EV Outlook Add-in will not exist until 9.0.1
http://seer.entsupport.symantec.com/docs/354858.htm
Latest EV client would be 8 SP 4... however against a 2007 SP 3 server, you will not get Offline Vault.
See : ftp://ftp.support.veritas.com/pub/support/products/Exchange_Mailbox_Archiving_Unit/276547.pdf for more information.
Personally if the server is EV 2007 SP 3, I would go with the latest 2007 SP 6 client
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