To remove unwanted letterboxing or pillar bars, try the following:
First, double check the original source file for letterboxing or pillar bars. Vimeo does not automatically remove hard-coded black bars from uploaded videos. If the source is letterboxed or pillarboxed, you can fix it by cropping or reformatting the file in a video editing program.
Check the video thumbnail. If you’re uploading a custom thumbnail image, make sure it does not include any black bars. Vimeo does not crop or edit thumbnails during upload.
If you still can’t get rid of those pesky black bars, give us a shout.