Yes, the Vimeotheque PRO will continue to work but you won’t have access to any updates or priority support. In addition, both Vimeotheque and Vimeotheque PRO should remain at the same version they were when the license key expired for Vimeotheque PRO.
If you continue to update the free version of Vimeotheque without updating Vimeotheque PRO, then, over time, the two plugins may become incompatible and you might end up with errors on your website and/or missing functionality.
The free version of Vimeotheque is constantly updated to include bug fixes and security updates and these code changes will eventually conflict with the outdated version of Vimeotheque PRO that was not updated.