The "Hello World" example is a really good example of why React, Vue, etc are better than something more minimal like this library if you're optimizing for speed. The page will show nothing until the script runs, which requires downloading the VanJS lib and the script itself. You could inline them into the page, but that gets seriously messy at scale. A modern React app that's using some serverside rendering for the initial page load will send the user a page of HTML that can be streamed (rendered as it downloads) so it'll display pretty much instantly (if you're concerned about styles you could inline for the important stuff and defer for rest). The React "Hello World" will be a lot bigger but it'll display
a lot faster on a slow connection, and it'll work if the user has JS disabled.
Obviously it's early days for VanJS and the authors could add all sorts of fancy SSR and clever scheduling, but if they do they'll lose the tiny size and end up making something that starts looking very like React (or more likely, Preact).
It's all a trade off. Sacrificing speed and complexity for size and DX is fine. You just need to be aware that's what you're doing, and neither is 'better' for all aspects of web app dev.