Scottish singer Sheila McKinley was part of the famous Les Humphries Singers in the 1970s. Found in the WWW.
Sheila appeared with denim waistcoat on promotional photos from around 1975:
She also wore the denim waistcoat while an appearance on German TV-show DISCO on the 26th of October 1974, promoting their single DO YO WANNA ROCK'N ROLL