Admittedly this may be unconvincing to you but to cite Wiktionary vagina plural vaginas or vaginae or vagin anatomy The passage leading from the opening of the vulva to the cervix of the uterus in female mammals zoology A similar part in some invertebrates botany A sheath like structure such as the leaf of a grass that surrounds a stem colloquial The vulva So no it s not wrong It s just colloquial non technical usage Besides the issue is usage not definition OED won t always have usage information and is more a source for definitions than usage Also it s not just me that uses it this way using vagina to mean vulva is widely used at least in my experience I imagine you ve heard plenty of people use it that way and even used it yourself Most importantly in most conversations if someone says vagina most people would understand that to mean vulva not the technical meaning of vagina listed in the article When usage is so widespread no matter one s opinion of it it can hardly be called wrong