incolor For instance, if the IPTC "description" reads "vimeo" then the video's URL would be in the IPTC "source" field.
That would work technically, but it's way too much of a "hack" to build in to Files Gallery, considering this also requires adding your own preview images and embedding IPTC data into each image.
There is another workaround solution, which seems much better. Following a post on Stackoverflow, someone built a website app that gets Vimeo thumbnails from the Vimeo ID, for example https://vumbnail.com/358629078.jpg. Using this, we could achieve the same as with Youtube (see my screenshot above). The only compromise is that you are making requests to a 3rd party website, and you can't be 100% certain it will always be up and working. For me, if I was using Vimeo, I would happily accept it as a solution.
The proper solution to get Vimeo thumbnails, would be to implement Vimeo API directly into Files Gallery. Although "proper", this is actually more clumsy, because we have to make more (API) requests, before we can even start loading the thumbnails. Anyway, I may consider this, but it will have to wait. I don't want to implement loads of individual external API's into Files Gallery, and besides, I have plans to implement basic "CMS" features into Files Gallery, where you can store data about items and create virtual links (for example Vimeo/Youtube) without having to depend on URL files.