I think RVW probably recognised it as a version of "The Grey Cock" himself. Here are a few more details, from here.

"The lover's ghost"
Text: English folksong, also known as "The Suffolk Miracle", "The Grey Cock", and "The Drowsy Sleeper"
Music by Ralph Vaughan Williams

Vaughan Williams had a fairly comprehensive knowledge of ballads, particularly those from broadsides. He often noted fragments from informants and, where he thought appropriate, added in verses from other sources (though his noted versions are accurate). This looks like a case where he didn't. I'll look tonight to see if I can find where he collected this one from.

You might also compare the RVW words with this version of the demon lover from Steeleye Span.

Cheers!
Ian