Based on MSDN documentation :

Vary by Query String Parameters

 

Specify a value as described in the ASP.NET 2.0 HttpCachePolicy.VaryByParams property documentation.

http://msdn.microsoft.com/en-us/library/aa661294.aspx

If you want to specify multiple querystring parameters, parameters separated by “;” or space didn't work.

We tried * and it worked. i.e. all URL with any querystring will be considered as separate URL and cached separately

-Ram