Green’s Dictionary of Slang

a sexually unrestrained person; thus a prostitute.

1771		R. King New London Spy 94: That pursey beldam has her emissaries [...] who look out for fresh goods [...] while the vestals about her are aplying perfumes, paint and patches.
1818		W. Perry London Guide xii: vestal ironical for an inconstant person.
1823		‘Jon Bee’ Dict. of the Turf, the Ring, the Chase, etc. 185: Vestal — ironically said of an incontinent woman.
1846		Bell’s Life in Sydney 13 June 3/2: Emma Stirling, a pert little nymph, was again introduced to the court by Martha Cadman, another vestal, who deposed that the defendant’s fist had been too intimate with her face.