We had [a lot of discussion in Moodle.org about the Vimeo image (actually, the player) not being "responsive" when inserted into a forum post.

The technique that I use to insert Vimeo is in ATTO, to click on the "Insert Video Item" tool, and then put my Vimeo video's "copy video link" URL (for example, this one https://vimeo.com/369899356) into the "Video source URL" and it properly shows.  In fact, it looks great, and just as it should!

However, this object (the video player's display) is not properly sized on narrow screens. It remains its static size.

Of course, when you insert an image into a forum post it does get sized correctly (meaning the responsive code is correct.)

This is not a big problem, in my opinion, but something that should eventually be fixed.  My guess is that the code that works behind the scenes might simply need to have the correct "responsive" code added, as some have suggested in the discussion.

As noted in the discussion, one might try to insert the raw HTML iframe code, but Moodle (correctly) strips this iframe code when using this technique.  And I am not suggesting that the solution should be to allow iframe code inserted.  I actually prefer using the "Insert video" tool.

As noted in the discussion, iframe code works fine for other Moodle resources, but I am focusing on forums.

Ref discussion: https://moodle.org/mod/forum/discuss.php?d=400225#p1614415