Varnish is very much like a programmable device. A VCL that works for one website can break the other. So it's important to know what you're up against in order to cook up the right VCL.

You could come up with a generic VCL that works for most websites out there but its cache effectiveness diminishes as you try to account for the most common corner cases. In fact, we did come up with such VCL. We distribute it with the cPanel Varnish plugin.

If you ever have a question or need a hand with Varnish/VCL drop me an email. I'll be more than happy to help out.