"sexual passage of the female from the vulva to the uterus," 1680s, medical Latin, from specialized use of Latin vagina "sheath, scabbard, covering; sheath of an ear of grain, hull, husk" (plural vaginae), a word of uncertain origin. Perhaps cognate with Lithuanian vožiu, vožti "to cover with a hollow thing," but de Vaan points out that "Obviously, this is a gratuitous proposal." A modern medical word; the Latin word was not used in an anatomical sense in classical times. Anthropological vagina dentata is attested from 1902.
1660s, "pod of the vanilla plant," from Spanish vainilla "vanilla plant," literally "little pod," diminutive of vaina "sheath," from Latin vagina "sheath of an ear of grain, hull of a plant" (see vagina). So called from the shape of the pods. European discovery 1521 by Hernando Cortes' soldiers on reconnaissance in southeastern Mexico. Meaning "flavoring extracted from the vanilla bean" is attested by 1728.
Adjectival meaning "conventional, of ordinary sexual preferences" is by 1970s, probably from the notion of whiteness and the common choice of vanilla ice cream; vanilla as figurative of a plain and conventional choice (without reference to sex) seems to date to the late 19c. as a noun, by 1940s (often plain vanilla) as an adjective.