Your code itself looks fine to me so I am surprised it does not work. The two likely culprits I can think of would be:
- the vimeo video itself for some reason is choosing a fixed size within the iframe
- Your element is in a parent group that does not have flexible layout setup, so the children are never repositioning.
Perhaps you could try adding a border to the HTML Widget element and see if the element itself is growing and the contents is not?
The only other thing is that you are using a HTML Widget for this, but since the code itself is an iframe then it is pretty much an iframe within an iframe. You could try pasting that into a Rectangle element’s Inner HTML instead. But this is more of an efficiency than a fix; your setup looks fine.