As part of the vault experiment, of the one thousand people who entered, one was a woman and the rest were men. The location and status of this vault are unknown.
Vault 69 is 68's counterpart which contained 999 women and only 1 man.
Behind the scenes
Vault 68 is a reference to the sexual position mentioned in the Kama Sutra, in which position 68 is for the sexual pleasure of the woman.