Russian or Soviet? Why the vietnam air force roundel would be painted on a russian air force airplane? I think every country has its own painting.
Dunno… afaik the window frame is just ordinary steel color; it has only a red star on the wings and tail, and the numbers are red also. What did you use as reference? Models on museums could be seriously different from the original ones, as they are generally reconstructed by people who don’t really care if the paintings are correct. I’ve googled “vietnamese mig 21” and all images that popped had red numbers and gray window frame. Anyway, art is art :D.