If you right click on the Vimeo thumbnail (within the video's settings) and select save as picture, the file requester will reveal the 9 digits you need to plug in to your RSS feed to use the existing thumbnail.
For those using Vimeo Pro I noticed that if you follow what the previous poster said, click the video Settings, then right click the thumbnail of the video in the top left, you right click > Copy Image location. It will give you something like:
I actually had old SD content which Roku gave an error "Thumbnails must be 16x9 aspect ratio". After resizing them in Photoshop on a whim I took the SD url, and again typed in ending it "800x450.jpg" and sure enough it resized it accordingly. Their servers must do the image sizing on their backend. Saves a ton of time. Just paying it forward in case anyone else is using them going forward and didn't know about this trick.