The better models can handle that prompt assuming there is an existing clean codebase and the scope of the task is not too large. The existing code can act as an implicit boundary.

Weaker models give your experience, or when using a 100% LLM codebase I think it can end up in a hall of mirrors.

Now I have an idea to try, have a 2nd LLM processing pass that normalizes the vibe-code to some personal style and standard to break it out of the Stack Overflow snippet maze it can get itself in.