If I slap a woman that way I would be hitting a variety of parts of her body. When you say vagina everyone knows what you mean, it is already part of the common lexicon to use the word in this manner, most people are simply annoyed by this type of pedancy. So, unless I need to give a small biology lesson when doing something slightly kinky to, or about, a sex organ, I am just gonna say vagina, or some other euphemism. Box, vag, pussy, etc.