Not to be immature or anything (interpret the question as you will...), but I was just thinking about how "V" is essentially a feminine symbol. The masculine symbol is usually the reverse, probably playing off the whole yin/yang thing. The structural similarities between the letters and sexual organs of both or undeniably similar, while somewhat simplistic (though its distant origin would suggest simplicity). Where did the word come from and did the fact that it starts with the letter 'v' influence its creation or propagation as a word? Then again, thinking on it....unless it came from English, then the word probably didn't start with a 'v'. Nevertheless, it would still be interesting to know how the word came to be.
Vagina comes from the Latin word vagina, which meant "sheath" or "scabbard". The reason for that etymology should be obvious. However, any visual similarity between the letter V and the anatomy in question is purely coincidental. The word ultimately goes back to a Proto-Indo-European root, which predates written language.