hey Eric,
I think it not related with the .htaccess rules, because I did tried just now temporary disable your site SEF and rename this .htaccess file, that issue still persists.
Can you consult with your webhosting provider regarding this and see how it goes?
I've created 2 PHP file from your Joomla root folder which using CURL to crawl to the video provider pages :
- Vimeo
https://vimeo.com/164864490
- Youtube
https://www.youtube.com/watch?v=ljakSDUjijc
If you manually access above links, it will render the correct page without any issue.
But if you crawl these 2 site using CURL from your server, it will return you this error
406 Not Acceptable, in order to see this, you can access this page URL
https://whoisontour.org/crawlvimeolink.php .
And it will crawl the correct content from the Youtube link
https://whoisontour.org/crawlyoutubelink.php , by right it should return Youtube video page HTML source.
Currently I am not sure whether your server got block this Vimeo video provider or not, you can consult this to your webhosting provider.