It's pretty easy to embed Vimeo's iframe in a responsive manner, so that the player changes in size.
BUT: Does Vimeo actually deliver the video in a correct size then?
First I assumed wrongly that they would check for bandwidth and progressively enhance quality or resolution, but Adaptive Streaming apparently is a thing for the future.
Do they at least downscale videos?
F.e. when uploading a video in 3000 px width and embedding it on a site in the responsive player, the video always seems to have a much lower resolution.
For adaptive streaming plays: yes. We set the maximum allowed resolution based on the size of the player and the device pixel ratio and it will adjust if the player size changes.
For progressive plays, the player defaults to SD. If the viewer clicks on the HD button, we will set their preference and the player will always use their preferred resolution when available. Plus and PRO members have an option to set the default resolution to HD, but that setting only matters for viewers without a preference.