FYI - Vimeo encodes up to 25p now - so you can rule that out.
Well, something went wrong on the the Vimeo conversion, that's for sure. I exported a 90 Meg h264 medium quality 25p file from my final Cut timeline (prores), and put that into Vimeo. I wasn't sure if Vimeo would recognize a Quicktime Prores file, and that one, for one minute, should be around a gigabite, so I compressed it. It played fine on my Octo 2.8ghz Macpro.