The root of this particular issue was Vimeo's failure to do this migration for their customers.

Vimeo OTT has a codebase written in Rails, whereas the main PHP application is written in PHP. At the time Vimeo acquired Vimeo OTT's codebase, the Vimeo OTT codebase was small — around 10,000 lines of Ruby. Rewriting that codebase inside the Vimeo PHP application would have been a tough technical challenge for the all-Ruby team, and they'd have likely lost some people along the way and missed out on some content deals, so they decided instead to maintain two separate codebases and two separate login systems.

The video-playback and video-storage infra has since been unified, but all the business logic is still siloed.