This is a quick crafty way of doing it, and also a way to pick a custom size.
I go here:
http://vimeo.com/api/v2/video/[VIDEO ID].php
Download the file, open it, and find the 640 pixels wide thumbnail, it will have a format like so:
https://i.vimeocdn.com/video/[LONG NUMBER HERE]_640.jpg
You take the link, change the 640 for - for example - 1400, and you end up with something like this:
https://i.vimeocdn.com/video/[LONG NUMBER HERE]_1400.jpg
Paste it on your browser search bar and enjoy.
Cheers,