The code is implemented on http://cup.wpcoder.de
I see that you're adding 'Accept' to the Vary header, which is great, but you're also varying the content based on the User-Agent, so that would have to be included in the Vary header for it to be correct.
Fortunately Chrome is now sending "image/webp" in the Accept header for image requests, (although not on html requests unfortunately) so you should be able to remove the User-Agent detection. Users will get the webp version for resources included on a page, but not if they request the image directly.
Sergej: Just checked your website, but in the latest Chrome Beta I see PNG's. Is this .htacess not working any more?
Sorry I didn't see your link in time as its expired. When I check in Chrome 35 Beta (osx) its still PNG's your site is serving.
@MusikDK, you probably see a .png files because that's what the URL in the page points to. But the server is serving an webp image "disguised" as a file with extension .png. It's an internal rewrite.