People write "vagina" often when they mean "vulva." The vulva is the outside, visible part of the genitals (labia, clitoris, vaginal opening) whereas the vagina is internal, the tube that connects the vulva to the cervix and uterus. Erotic authors often make the mistake.
HOWEVER, "pussy" is used for both the vulva or vagina. So if you use "pussy," you can't make that mistake.
Okay, so you all weren't dying to hear that tidbit of information.