Too Long; Didn't Read
There are plenty of ways to use <a href="https://hackernoon.com/tagged/viewport" target="_blank">viewport</a> units, covered in <a href="https://css-tricks.com/fun-viewport-units/" target="_blank">many</a> <a href="https://www.smashingmagazine.com/2017/05/fluid-responsive-typography-css-poly-fluid-sizing/" target="_blank">good</a> <a href="https://css-tricks.com/between-the-lines/" target="_blank">articles</a>. But the use cases are mostly limited to specific elements. In theory, you could use viewport units to scale the entire website layout relative to the viewport width. Of course, this does not work for most websites. But there are edge cases where you need a layout that is fixed, but also relative to the viewport width.