I think it is a smart process in using vray sun with the vray dome light combined with HDRI in texture slot
scene is totally illuminated with vray dome light and HDRI ..
vray sky acts as HDRI reflection map when it is placed in the environment slot ,so it is a background and also light the scene ..
the important thing that I believe in is that it should not be a conflict between HDRI light map and the light colors of the vray sky
i.e for example putting sun set HDRI light map with a set of vray sky & vray sun at noon time of the day .
