The issue-
The setting (which was on on your group) is designed to offer an alternative to players embedded in an iframe that do not have permission from the containing iframe to “allowfullscreen” (that is the parameter name for your reference). When you have a moment, can you reach out to Wix support about how to allow fullscreen within their Iframe embed elements?
This forum is for issues with code on your page, you are best suited going through Wix Support
https://support.wix.com/en/article/contacting-wix-support
As for adding videos, Wix already have lots of support pages for videos too.
https://support.wix.com/en/the-wix-editor/video
You will find info about full width videos here.
https://support.wix.com/en/article/adding-videos-to-your-wix-site
Or about adding Vimeo videos here.
https://support.wix.com/en/article/adding-a-video-from-vimeo
Notes: Make sure that your video privacy settings in Vimeo are set to Anyone can see the video and the embed settings are set to Anywhere.
Vimeo uses adaptive streaming. This means that the video quality depends on the bandwidth available for the device viewing the site.
As for trying to change the code used through an embedded iframe, well this is something that Wix does not support as it is not code that is tried and tested by Wix. If you have issues with the html code given from Vimeo, then you need to go back to their own customer support section and ask for more help too.
https://support.wix.com/en/article/guidelines-and-limitations-of-the-html-code-and-embed-a-site-elements
Important: We are unable to provide support for external HTML codes that are added to Wix sites as the codes have not been tested by Wix.We recommend testing your code before you publish.
On WIX, and other similar website platforms, the full screen iframe option is not supported. To get around this you need to embed your iframe using HTML code as a work around. If this is going to be a problem, you can make an image load the media in a separate browser tab. This work around will place a small link beneath your code that will open a new tab to a full screen iframe.
However, this is something that you should be able to obtain from Vimeo themselves or you will have to simply Google it yourself to get the code for it from another site and configure it for your own html code.