Just to clarify, how would the EV OWA extensions affect users using Outlook on the client machines? EV OWA and Outlook do not relate to each other, EV OWA would only affect users access their vault using Exchange OWA. If you have users having issues with Outlook client then it wouldn't be related to the EV OWA exteionson on the exchange CAS servers. You would need to check the user's outlook client add-in and various other basic troubleshooting.

Back to the EV OWA issue, did you say you installed the EV plugin on the CAS servesr? Do you mean you reinstalled the EV OWA extensions for exchange 2007 on your CAS servers? You need to clarify exactly what you have installed on your CAS servers and what is status of your web.config on your CAS servers. Was EV OWA working before the exchange updates? If it did then you need to check the backed up web.config.

To perform enterprise vault functionality within outlook you need to install Ev outlook add-in on the client machine. Only when you have a user enable for archiving and oultook add-in install user can see the Ev icon in the tool bar and he can perform enterprise vault related task like store in vault , restore from vault , Ev serach and archive explorer etc. 

As suggested by Chau, installing OWA extension on the Cas server would allow you to see the EV option when accessing emails via OWA.At the moment the details you have posted is from the EVORT, you may refer the following document to troubleshoot. http://www.symantec.com/business/support/index?page=content&id=TECH154553

Do you mean your clients are using Outlook Anywhere (RPC over HTTP) and unable to retrieve archived items? 

If so what errors do you get on the client (please can you screenshot) and also perform a client trace and upload both.

How to perform a client trace for EV within Outlook....
http://www.symantec.com/business/support/index?page=content&id=TECH38096