I was thinking at HTML level [document]. Be it the HTTP headers or in the head section of the HTML document. Probably it is equivalent to the srcset proposal. The advantage is that it has no side effects with current browsers [i.e. the IE Vary problem] nor with current proxies, because the URL is different [cacheability without problems] and because proxies do not have to learn new tricks, such as vary: accept.
On the other hand, I concede that the “vary: accept” is slightly simpler from author’s point of view. I might be wrong, but I would guess that IE <= 9 would have problems with it, i.e. the images are not cached by those browsers.