"Vim is, without question, the worst C codebase I have seen."

From the example you give, I agree it must be bad. But if you want to see something worse, check out PHP. No, not the billions of programs written in the PHP language (which are, indeed, almost all terrible), but the C source to the PHP interpreter.

Obviously PHP was invented by someone who just doesn't care about creating a decent programming language. Everyone knows that; it has been pointed out so many times that it's a cliche. But if you look at the source to the implementation, you will find that it was developed by someone who is so incompetent that whether they care or not is beside the point. Dealing with that codebase (and thinking about how successful PHP has become) made me question my will to live.