Vimeo Showcases do work with Roku channels that are specifically programmed to communicate with the Vimeo API. For example there are quite a few developers successfully using Vimeo on channels built with Instant TV Channel.
Vimeo DID NOT end support for OTT apps like Roku channels. They only ended support for their own Vimeo-branded app on various platforms. Vimeo is very much alive & well on custom SDK Roku channels.
You do not need to move your content off of Vimeo in order to convert from Direct Publisher to SDK. And moving videos to S3 so that you can stream directly from Amazon S3 or Cloudfront is usually the wrong thing to do, as AWS bandwidth is quite expensive compared to other solutions.