One morning spent comparing the etymologies of "vulva" and "vagina" (one of the many Stuff Mom Never Told You job perks), I made a side note discovery. The word "vagina" is far more closely related to "vanilla" than "vulva." In fact, both come from the same Latin root, which just so happens to be "vagina" as well.
"They're word sisters!" I exclaimed to my dog who was napping at my feet. He didn't even stir, but I wasn't offended. I'm constantly bombarding Buddy with new Stuff Mom Never Told You facts I learn, so it's understandable that he sleeps through them from time to time.
But back to these sister words (not to be confused with Sister Wives). Both are derived from the Latin word "vagina" meaning "sheath, scabbard." In the early 17th century, "vagina" in the sense of a woman's internal canal extending from the uterus to the vulva (or penis sheath?) came about in medical literature. Then, in the mid-17th century, some Spanish guys stumbled across the vanilla plant in Mexico, and apparently it reminded them of...a vagina.
Mental Floss explains:
Of course, the irony of all this is that "vanilla" these days is most closely related to "vagina" when referencing ordinary, heteronormative, penis-in-penis-sheath sex. But when you consider their shared etymologies, "vanilla sex" takes on a completely different, lady-lovin' meaning. Nor will "vanilla ice cream" ever sound quite the same.