I've never had any issues with any Vimeo videos on any computer except for an iPad. I keep trying on different computers but have yet to have any problems. It intrigues me that so many people complain, yet I've found it impossible to encounter.
Vimeo claim to have sorted i-device playback. Have you tried the app? Found that to work well with Vimeo
Try running it with Google Chrome.. It is way less combersum that Firefox and relies on less plug-ins.
I like Google Chrome for streaming videos. It seems to work better than IE and Firefox.