In some regional vernaculars, "y'all" is singular and "all y'all" is plural.

Interesting. Here in North Carolina, "y'all" is generally plural, whereas "all y'all" is also plural but more emphatic, as in "I really do mean every last one of you", or sometimes simply to indicate a stronger emotional content, especially when angry.

South Carolinian here that spent a lot of time in Raleigh & Charlotte and is now living in Austin. "y'all" can be both singular and plural, depending on the context. "All y'all" applies to larger groups (sometimes including people that aren't present), as well as with additional emphasis.

"All y'all come by for supper" meaning you and your close family. And maybe even some other people as well, if there's room at the table for them.

Sounds like a slippery slope.

And now we're back in the "use the plural when talking to a single person to be formal" that started this whole thing.

Native Texan here. I've never really heard it being used as a form of "you" and that just seems silly to me. "Y'all" is definitely plural.