Actually I think "complexity" is probably not quite the right measure. You can vibe code very complex things, as long as they are in the "easy for AI" zone - clear goals, ideally non-graphically, ideally there's a specification or even better a reference implementation, best an automated way of checking correctness. CAD is about the worst case (and very complicated).

But you could easily vibe code, e.g. an H.265 decoder, even though it is super complex.