When serving a response cached by the server, IIS 7 is stripping the Vary header - caching

Question

I’m using a custom framework for bundling stylesheets and scripts. (I.e., these are dynamically generated responses, not static files.)
The response for the initial request, when the response is being generated for the very first time, includes these headers:
HTTP/1.1 200 OK
Cache-Control: public, no-transform, max-age=31536000
Content-Type: text/css; charset=utf-8
Content-Encoding: gzip
Last-Modified: Mon, 25 Aug 2014 18:15:50 GMT
Vary: Accept-Encoding
Date: Tue, 09 Sep 2014 16:19:36 GMT
Content-Length: 3126
Now that the response above has been generated and cached by the server, subsequent requests for the same stylesheet are responded to with these headers:
HTTP/1.1 200 OK
Cache-Control: public, no-transform, max-age=31536000
Content-Type: text/css; charset=utf-8
Content-Encoding: gzip
Last-Modified: Mon, 25 Aug 2014 18:15:50 GMT
Date: Tue, 09 Sep 2014 16:20:00 GMT
Content-Length: 3126
Ignoring the new Date value, the headers are identical with the obvious exception of the missing Vary header.
One nasty consequence that I’ve seen in the wild is that if the very first response generated for a given asset is not compressed (due to the corresponding client not supporting compression), then the server caches that non-compressed response and serves it for all subsequent requests to all clients.
Any idea how to have the server retain the Vary header for cached responses?
I’m using HttpCacheability.Public for these responses. I can avoid the issue by using HttpCacheability.Private instead, but I’d prefer to allow the server and proxies to cache responses.
Some reading has led me to believe that IIS can’t do “kernel caching” if you vary by encoding. But I’m not sure if that means I can’t cache on the server at all or if it just prevents a special kind of server-side caching.
Update:
I was originally using the following to set the Vary header:
response.AppendHeader("Vary", "Accept-Encoding");
I tried a different method of specifying it:
response.Cache.SetVaryByCustom("Accept-Encoding");
That caused Vary to never be emitted. Not even on the very first response.
As a last resort, I’m also considering using:
response.Cache.SetNoServerCaching();
That causes the Cache-Control header to still specify public (so that proxies can still cache), but prevents the server from caching.

Answer 1

In line with the update I made to the question, I tried yet another way of specifying the Vary header:
response.Cache.VaryByHeaders["Accept-Encoding"] = true;
…And it fixed the problem. Responses now retain the Vary header across requests from multiple clients and are also cached by the server.