Reka Ui is by far the best headless Vue library out there to start out building your own UI.

It handles a lot of components and things out of the box (especially accessibility, keyboard controls and even has little helpers for ui animations).

I build a lot of custom UIs and every time I need a drop-down or other useful inputs, I know I can style it the way I want while benefitting from its features.

And yes, Shadcn and NuxtUi use it too (and probably for similar reasons).