Configuring varnish for many different domains with similar content

[Hip Hydra]

Hi, I'm running a network of web proxies, where the domain of each proxy 
is different, but any websites a user goes to will have the same content 
regardless of the proxy domain.

For example:
http://proxy1.com/visit.php?site=22abc862ce4
http://proxy2.com/visit.php?site=22abc862ce4
http://proxy3.com/visit.php?site=22abc862ce4
...

These URLs all go to the same site. However, proxy1.com, proxy2.com, 
etc. have different base files. Only requests that go through visit.php 
are the same if the site variable is the same.

Is there any way I can optimize Varnish to cache the data files more 
effectively?

Many thanks!
John