They distribute a special Blender build for VRay users only, the GPL permits the creation of special builds like that (without source access) as long as you don’t make it available on a public website.
The issues it brings up though is the fact that Chaos Group has to do extra work just to make their engine usable for Blender users and a person using a special VRay build cannot make use of data created in other special builds (say you have one for Substance support and one for Realflow simulations). Ton himself does not like this situation, and the only way otherwise (if you want fast and deep integration) would be a sort of (C++ based) non-GPL API that could act as a bridge for proprietary solutions to hook to (which is unlikely).
This makes it all the more important to try to make Blender into something that excels in all areas of the pipeline (it brings the dual benefit of allowing anyone to make professional-grade CGI without a huge financial barrier). That last part would be more important now because of the major production apps. going to a rental-only model.