Could you use the opengraph image as a thumbnail? 
For example, this URL: https://vimeo.com/795669787/0763fdb816
Returns a og:image value of: https://i.vimeocdn.com/video/1602645154-56e0101b545c38ae03e9bc65ae12c3bcc37cd8ea127d359c0d24cf8ab31f8675-d
Which (correctly) displays as:
