Hi Ozi, good question!
A couple things I’ll point out real quick is that other community members won’t be able to see the preview of your LP (I can just because I’m on the Unbounce side), so if you’re able to share the published URL you may be able to get more responses from other Unbounce customers. But I’ll do my best here to help out.
Something that we recommend for video backgrounds is embedding them in an HTML widget instead. This option is far less restrictive in terms of responsiveness. Here’s some documention on the Vimeo side of things for embedding your video.
And here’s the documentation from the Unbounce side when you’re adding an HTML widget to your page.
Then just adjust the properties of the custom HTML element so the sizes match.
I’ll keep an eye out. It’s possible other members in the community have found a way to adjust their video backgrounds using custom scripts. Hopefully this provides a little help for now.