Hi,
For me it works, like on
http://www.formz.com/products/vray/V-Ray_Documentation/index.htm#t=Clipper.htm&rhsearch=clipper&rhhlterm=clipper&rhsyns=%20
explained. So basically you create your main object and then your clipper-object. You have to assign the clipper object, by clicking on it with the Vray clipper tool.
And you can set materials for main object and clipper object differently.
But for me to show the material of the clipper on the clipped object, the "Use Object Material" should be unchecked(in the options of clipper tool).
Hope that helps!