At some point in 2021, Vimeo has changed their customers video thumbnail URLs which resulted in old URLs displaying colored bars instead of the correct image.
To update and fix your Vimeo video thumbnail URLs, please download the latest version of the WPVS Vimeo Importer plugin from your Downloads page:
If you already have the old version installed, you can uninstall and delete it before installing the new version. This will have no effect on your existing Vimeo videos.
After installing the new version, go to Tools -> Vimeo Importer in your WordPress Dashboard. In the top left corner is a blue button labeled Update / Fix Vimeo Thumbnails.
Once on that page, press the blue Update Vimeo Thumbnails Now.
Important Note (version 1.0.1 only): Will update ALL Vimeo Video thumbnails even if they have custom thumbnails set within WordPress. If you only have a few broken images, it's best to manually upload new custom thumbnails for those videos, instead of running this updater.
Version 1.0.2. Only Vimeo thumbnail URLs will be updated.