Dear Tom,
do you mean a particular video or all of them? I’ve tried watching e.g. the video in this post: http://www.tomgeraedts.com/?p=41 in full screen and it works fine (when I press the “full-screen” toggle between the HD and the Vimeo logo on the video control panel). I’ve also checked the embed code, it has the “allowfullscreen” parameter as well. ..Or have you already solved the issue while waiting for my reply? :)
Best,
Andrew | Satori