It would depend on the size of the project, and the willingness of the vibe coder to improve and learn.
- Small project: I would fork it and make it mine.
- Large project: not worth contributing.
- Junior dev: I would maybe help him and teach him that AI is not the answer when you want to learn.
rewrite it yourself
Rewriting is almost never a good idea, but you can still fork it, and slowly refactor and clean it.