There is a fine line if any between "re-encode" and "convert".
I suspect that your uploads to Vimeo end up "converted" as H.264 flash file with AAC audio.
I read somewhere that Vimeo converts the upload to "industry standards and compatible formats"
To add to the mix is "Flash" versus "HTML5"
Have you been uploading your video to Vimeo? If so, good or bad results?
The results have been very good. Great video quality and no delays in playback.
I will keep looking to see if anyone has put online the exact details of the Vimeo "conversion" for the uploaded videos.