The lazy-video-wrapper that you see is a part of a change we made three weeks ago in the way we handle embeds. Vimeo embeds are now lazily loaded, which means that the iframe is only appended on demand.
https://github.com/discourse/discourse/pull/20722
Missing thumbnails are a known limitation with unlisted Vimeo uploads. There may be a workaround, but we are still investigating it. For more information, please refer to