I read several older forum posts asking for the ability to change playback speed. I don't know if Vimeo changed anything, but it seems to be implemented now.
When you hover your mouse over the video, the bottom left of the video should have a gear icon (between the volume and fullscreen buttons). If you click that, a menu opens and you can choose Speed and Quality. You can then change the speed to 0.5x, 0.75x, 1x, 1.25x, 1.5x, and 2x.
Hopefully this helps someone!