Green’s Dictionary of Slang

(Aus.) a Chinese greengrocer.

1902		Bulletin (Sydney) 26 July 36/2: Cycling down Sydney Road (Melb.), t’other morning about 5 o’clock, suddenly espied, in the middle of the tram-track, a vegetable John suspended by a rope around his neck from the push-bar of his two-wheeled truck, toes just touching the ground and face the color of watered ink.
1982		N. Keesing Lily on the Dustbin 118: The horse of the chinese ‘vegetable John’ pulled a distinctive open cart.