Also, isn't Vanced closed-source?

I find the people behind it slightly sketchy.

It's YouTube reverse engineered so it seems a bit difficult to open source something like that

Newpipe is also youtube reverse engineered, as is yt-dl/dlp. No need for closed source.

Vanced is a patched official youtube app.

But do they distribute the source of the patches?

I searched some time ago and didn't find any, so they probably don't. They do distribute the sources of their manager app and their fork of microG tho.

Yes but NewPipe is a completely from-scratch implementation and has some unique features like 3x speed or downloading videos and audio. The NewPipe UI is not as good however, so it depends on what tradeoffs you want to make.