Hi @NCGELibrarian ... thanks for the question!
Vimeo handles embedding with a differently formatted URL.
For instance, this video of an Esri commercial has a URL of:
However, the URL in the embed code is formatted like this:
It appears that Vimeo does not allow for embedding using the standard URL that we use in a web browser. Vimeo only allows for video embeds using their embedded URL construction.
You'll need to update your video's URL of the Document Link in ArcGIS Online.
Go to the Settings of your Document Link, and update the Data Source URL.
Once you save, and reload that video page in Hub, you should see the video rendering correctly.