Will you please either link your blog to your username or post the complete URL for the blog in question starting with http:// when you post to this forum?
To link your wordpress.COM blog to your wordpress.COM user name > Dashboard -> Users -> Personal Settings -> then scroll down to ‘Account Details’. In the spot where it says ‘website’ fill in the complete URL for the blog in question starting with http:// your blog and save the changes.
you cannot at once post the video, the vimeo videos are in the processing queue and take some time to process, it might take one day. After that you can use the vimeo video shortcode. Read the document pointed by the link @timethief provided