"The downside of vibe coding the whole interpreter is that I have zero knowledge of the code."
This is exactly the problem. When I first got my mitts on Claude Code I went bonkers with this kind of thing. Write my own JITing Lisp in a weekend? Yes please! Finish my 1/3rded-done unfinished WASM VM that I shelved? Sure!
The problem comes, that you dig too deep and unearth the Balrog of "how TF does this work?" You're creating future problems for yourself.
The next frontier for coding agents is these companies bothering to solve the UX problem of: how do you keep the human involved and in the driver's seat, and educated about what's happening?