I'm a Senior Web Applications Developer located in Texas. I have 15 years of experience in integrations and automation, and I have 25 years of experience in web development, front- and backend.

The Vimeo OTT SDK has hooks for getting and setting video volume as well as knowing when captions are enabled as well as which captions are enabled. Adding this functionality should be as simple as altering the template for video playback and storing it as cookie or session data and would be covered by your existing data and privacy policies.

I will gladly donate my time and energy to solving this problem as a longtime fan and subscriber.

On another note, the support staff that handle help tickets from the site are very friendly and nice.

Edit: Thanks to some additional info from u/abridgma it turns out that fixing this would require Vimeo to change their OTT platform and is not something that can even be approached through the interfaces provided to Dropout. In order for Dropout themselves to fix it, it would require a massive architectural overhaul of the entire digital side of Dropout.

Fortunately there's hope, u/KurtThiemann has an interesting browser extension  here to solve this issue for many users and add additional functionality.