Enterprise Vault uses port 80 and 443 to communicate with Outlook Web Access (OWA).  The Enterprise Vault buttons will not appear if there is a firewall between the Client Access Server (CAS), Exchange Mailbox Server or the Enterprise Vault Server that blocks communication through these ports.   The port in question will be dependant on the current method configured to connect to the Exchange Virtual Directory.  By Default, this connection uses SSL (HTTPS), or port 443.

Confirmation:
Enable OWA Diagnostic logging on the Exchange 2007 CAS server by adding the following line in the web.config:
Step 1: From the Exchange 2007 CAS server, navigate to the "Program Files\Microsoft\Exchange Sever\Owa and open the web.config with Notepad.
Step 2: Enter the following key in the "<add key>" section located 3 lines after "example"

<add key="EnterpriseVault_LogEnabled" value="true"/>

Step 3: From the Exchange 2007 CAS server verify that the logs were created in the directory listed below.

"Install_path\Program Files\Enterprise Vault\OWA 2007\logs:"

Step 4: Look for the following errors:

[WebDAVRequest::Send] Exception sending WebDAV request: System.Net.WebException: The remote server returned an error: (400) Bad Request

[WebDAVRequest::Send] Exception sending WebDAV request: System.Net.WebException: Unable to connect to the remote server ---> System.Net.Sockets.SocketException: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond <IPAddress>:443

Note: <IPAddress> will be the Exchange 2007 Mailbox Role Server.

Resolution:
1. Verify the server the above IP address belongs to.
2. Disable the Firewall, or allow two way communication on port 80 and/or port 443 between the CAS Server(s), Mailbox Server(s) and/or Enterprise Vault server(s) found in the above log.
3. Clear Internet Explorer temporary internet files, and attempt to log onto Outlook Web Access (OWA) again.