Quote Chipp - "I'm not sure I understand your point about sun changing to a shadow map when a volumetric model is used for the sun. In my version of Vue, it maintains itself as a ray-traced light and shadow when set to 5. You might want to check it out for yourself."

Chipp, please do this experience. Open Vue with the default atmosphere. Edit the sun and verify that shadow map is not on. Ok, dont change anything, just close that dialog box. Now, with the sun selected, make the sun volumetric in the object properties panel. Now, edit the sun again and check the shadows tab. In my case, the shadows automatically turn into shadow map. It's not even necessary to make the shadows soft, I was wrong, all it takes is for you to turn the sun into volumetric.
This behavior is even documented in the manual, I found it now (p260). It's a feature, not a bug. :-)