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 black bars. If your thumbnail does not contain black bars, make sure that the aspect ratio of your thumbnail matches the aspect ratio of your original video file. For example, if your video is 16:9, your thumbnail image should also be 16:9.
If you still can’t get rid of those pesky black bars, give us a shout.