The trouble is "folk" has a range of meanings. That's where the disputes arise as to which meaning should be sanctified - a broad one equivalent to vernacylar here, or a more restricted one, and which restricted one, 1954 or whatever. "Vernacular" doesn't invite that.

I suppose we still might get arguments about whether some song that nobody has heard of which gets resurrected would count. We like arguing...

I suppose this use of vernacular is analogous to that in architecture, where it's about buildings put up without an architect interfering with the way buildings are generally put up in that part of the world, even if it's an architect who might design a particular building.