You need to click the full screen button on the site to see it in HD. From their FAQ
Why is my video only 640×360?
HD video is so big that it can’t fit in the width of a regular web page, so we show it at half size. To see the whole image, go into full screen mode by clicking the full screen button in the lower right of the player. The native size of Vimeo HD files is 1280×720.