The other commenters seem to think we are asking about content that WE PRODUCED. But we are seeing these thumbnails on everything, even Vimeo's "suggested content" and anything in Vimeo.
(This is only on the ROKU app for Vimeo. In a browser like Chrome, for example, we see thumbnails.)
It sounds like there is no solution unless the content creators do something to their Vimeo video. (Though so far I haven't seen anything in the ROKU app for Vimeo show anything but color bars.)
We’re sorry to hear that you’re encountering issues with thumbnails not loading in the Roku app. We are aware of the issue and our engineers are working to address it.
Recently, our engineers implemented an update to increase the security of video and image assets on Vimeo, including thumbnail images. Unfortunately, this had the unintended effect of preventing video thumbnails from appearing for some Roku Vimeo app users like yourself. As mentioned, our engineers are working to address this issue. However, since Roku is a third party device that requires additional approval and testing ahead of updates released to customers, this process may take some time. We do not have a target date for when this will be completed, but we will release an update as soon as it’s available. We apologize for this inconvenience and are happy to provide any other assistance needed.
I don't think Vimeo intends to do anything about this. Maintaining Roku-compatibility does not seem to be a big thing for them. The thumbnail artwork is still available, it's just that Vimeo changed all of the URLs. For all of the older Vimeo-sourced channels that I'm familiar with, it was necessary to regenerate all of the thumbnail/poster URLs. Vimeo also broke their MP4 URLs a few months ago, I'm not sure if that's been entirely "fixed" yet or not. So when regenerating your thumbnail/poster URLs, you might want to replace any MP4 URLs with Vimeo HLS or DASH URLs at the same time.
It's incredibly bizarre that it's gone on for so long.
I think there's a bit of a circle here: it's likely not being fixed because not many people use Vimeo on Roku, however, the reason not many people use Vimeo on Roku is that it's not been fixed! Few people complain about it because they just stop using the app and go elsewhere. Neither Vimeo nor Roku seem to care.
It has now been over one year since this problem began. I still use Vimeo on my Roku, but not as often as before. As, MattyB noted, I do not think enough people use Vimeo on Roku for the problem to be fixed.