just want to post some info about my problems with foggy vivid weathers. i think i found the culprit: if sr godray is enabled and sunglare enabled = 0 then fog in vivid weathers turns to a blocky ugly mass of color. switching from 0 to 1 solved this problem. i was not aware of this and was close to kick vivid weathers. is there any reason why this parameter messes fog apperance up?
sorry for the late answer. the colour problem seems to be common banding problem in skyrim and depends on the brightness settings in darker environment. a debanding shader setting (in this case with reshade) helped my a lot to reduce this problem.