Stop searching for that button. It isn't there. You'll need a piece of software like this one called Free Vimeo Downloader to have those videos downloaded easily.
Not only will it download those videos but it will also give you the possibility to have them downloaded as many different formats like AVI, WMV, MPG2, MP4, MOV and other compatible with iPhone, iPad, PSP or PS3.
In addition you'll be able to change output settings like aspect ratio, video size or codecs.
In short, you'll only have to set how you want that video to be downloaded and enter the URL of the video you want to have downloaded and click the 'Download' button.