you prolly have the wrong markup code.. if you are using

(.*vimeo.com/)(\d{5,20}) as a regex, then you need to use the 2nd capture (\d{5,20}) in your markup "\\2" and NOT "\\1"...

if you do \\1, then the regex will be very 'greedy' (regex speak) and evidence itself by what you prolly saw..

Linky Poo™ for a very useful proggie that explains regex-speak