Green’s Dictionary of Slang

(US) a woman willing to have sex (or at least go out) with servicemen for patriotic reasons.

1948		J.V. Maresca My Flag Is Down 171: Once or twice a V-girl and once even a whore comes over to talk to him.
1957		A. Coppel Night of Fire and Snow 46: ‘A rather attractive girl, wouldn’t you say, Allie?’ Billy snorted. ‘I know a V Girl when I see one.’.
1984		(con. 1940s) in S. Terkel Good War 244: I remember young girls in three, arm in arm, shuffling along the pavement in their saddle shoes. They were called Victory girls.
2006		(con. 1940s) R. Hughes Things I Didn’t Know (2007) n.p.: ‘Victory Girls’ who hung around the pubs [...] of Melbourne, picking up American G.I.s.