Many people have had playback issues in HD. In my case - use a 4 year old low spec acer laptop, I used to have horrible framerates, about 1 fps with HD on. I've been bothered by this for a long while, especially because I used to be able to watch HD just fine. The Vimeo staff are aware of my problem, but they couldn't help me either.

But now I have found a solution for the problem all by myself! It is actually quite simple. Here are the steps I took:
1) Download the official Adobe Flash Removal tool here: kb.adobe.com/selfservice/viewContent.do?externalId=tn_14157
2) Cloze down ALL your browser windows.
3) Run the removal tool
4) Reboot the computer
5) Go to Vimeo, and from there, reinstall the Flash Player.
6) Reboot again.
7) Enjoy watching Vimeo in HD in great framerate.

I get fluent video mostly for 1.40 minutes, and then it gets choppy again. If I then pause the video, go to Windows Video properties, change the resolution and put it right back, and only then click on ok, the choppyness is gone when I return to vimeo.
So I guess it is a combination of faulty flash programming, and bad video drivers. My portable pc is acer brand, and I'm running the latest driver, but I guess it sucks big time!