The proxy server
provides access to most licensed resources. However, some programs do not work with the proxy server. These require VPN for off-campus use: ARTstor (Macintosh only), JSTOR (Mac OS X), Luna Insight, the java client in UC Image Service, and the Connect function in EndNote.
Another difference is that the proxy server requires a one-time change in your web browser settings, then prompts you when you need to log in. VPN is a "client" software program that you download and install on your computer. You must run the VPN client each time you want to use licensed resources.