Yes, I agree with DMM - there is slight banding at 32 bit. It's a little easier to see if you mask out the bright central band with your hand. As Angel raised the question, presumably it's not just a JPG artifact and is evident in the original image too. I have seen similar (and worse) banding in Terragen, but not in a Vue sky before. My only thought is have you tweaked the gamma either in Vue or in post? I have found that doing so in post (or using PSP's Clarify filter) on a Terragen render can create very obvious bands in large blue graduated areas of the sky that were barely if at all noticable originally.