Yes, you are right with ["vimeo(dot)com/
(...)/
(...)"] the green part being a video's numeric identifier and the purple part being its alphanumeric access key. The thing is that something must have happened to XenForo with its latest update as I had been able to embed restricted videos just fine before it. Now it is not possible.
For instance here is my post from July 20th, 2021:
https://forums.taleworlds.com/index...pansion-michael-the-brave.445104/post-9723723. It has a video which still is displayed error-free and - upon clicking on it - redirects to the site, regardless of its restricted status:
https://vimeo.com/576927599/97a73bb97b. It is a proof that something has happened to XenForo itself.