Microsoft ISA Server 2006 can be used to secure access to OWA 2007 servers by using Web publishing rules to make Exchange 2007 OWA Web site available on the Internet.
Figure: Access to Enterprise Vault using ISA Server 2006 shows how ISA Server 2006 can provide access to Enterprise Vault.
In addition to publishing the OWA 2007 Web site, you also need to publish to clients the Enterprise Vault Web Access application. This is because Archive Explorer and archive search client requests attempt to connect to the Enterprise Vault server directly.
See the following articles for detailed instructions on how to configure ISA Server 2006:
http://www.microsoft.com/technet/isa/2006/deployment/exchange.mspx#client
http://www.isaserver.org/tutorials/Using-2006-ISA-Firewall-RC-Publish-OWA-Sites-Part1.html
http://www.isaserver.org/tutorials/Using-2006-ISA-Firewall-RC-Publish-OWA-Sites-Part2.html