will "Vibe Coding" increase or decrease the quantity of open source code?

Co-founder, Scout Game. 
https://scoutgame.xyz

both. increase in production increases learning so the high quality projects with strong core contributors setting good guardrails will go up, lots of throwaway dead projects. I don't think maintainers of high quality repos will just merge willy nilly. never have. lots of tools, and checks will be built to automate clean code and robust organization.

that being said, you can also have lots of oss code that gets high usage and lands up being poor quality but it has pmf and vibes with the audience.