Vanilla CSS means CSS without any framework, pure CSS.
When I started a career in web development, we are making webpage using table tags. Surprised!?! Yes, It was.
Then I was making webpage using float. At that time Bootstrap and foundation CSS frameworks came and all web developers/designers started to use them.
Now, CSS has variable, flexbox and grid box features.
We can make responsive layouts instantly with flexbox and flexbox is supported by major browsers. We can see it at https://caniuse.com/?search=flexbox. The flexbox is explained sufficiently on CSS-Tricks https://css-tricks.com/snippets/css/a-guide-to-flexbox/.
Nowadays, Web Designers/Developers are using Tailwind CSS but It makes the webpage design more complex. I believe in simplicity for coding.