For anyone using Vimeo hosting platform, yet likely hand coding json, check your channel & thumbnails. Seems they did a changeover which gave new urls for thumbnail images, and they old ones are now defunct, displaying only color bars. I had some trouble with the new ones not always showing either, and reached out to Vimeo support. There's a thread in the regular developers section as well.
Otherwise, if you set your video to public, and click its shorter public/normal url, you will see a little thumbnail of it plus other videos on the bottom right. Right click it and select 'Open Image In New Tab" and it opens a bigger image of it. Just add _800x450 to the end of it to resize it as 800x450 or whatever you need (Direct Publisher needs that sizing I believe)