Green’s Dictionary of Slang

1. a stupid person.

1965		W.C. Anderson Adam M-1 203: Don’t you shame on me, you limp-legged vegetable.
1986		T. Winton That Eye, The Sky 39: Ya old man’s a vegetable, and ya mum’s a pisstank.

2. (US gay) a lesbian [fruit n. (2)].

1972		B. Rodgers Queens’ Vernacular 71: any gay woman [...] vegetable (camp: because they’re not known as fruits).
1988		H. Max Gay (S)language.