I didn't get out of the post a focus on definitions as the main point. I got out of the post a concern about massive technical debt, which is the hallmark of both legacy code and vibe coding. And no, I'm not offering a definition.
I wouldn't get too persnickety about definitions. There are lots of arguments about what 'legacy' code means. you are throwing around the term quite a bit without really giving any definition at all. 'Code that nobody understands' is fine as a quick and dirty pointer.
Martin Feathers, in the most popular book on legacy code, defines it as code without tests. This is not a good definition at all, but it shows just how hard it is to define it. This is not meant as an attack on anyone: I just think some flexibility and slack is called for here.
You could easily write an long white paper trying to define 'legacy code', and it would be a lot less interesting than the target article here.