There are words that are commonly misused

"Commonly misused" is an oxymoron.

It's a commonly misused word but it's definition hasn't changed because it's used correctly just commonly enough (among actual doctors and people who are educated on things beyond just "hey! I don't know what words mean so instead of getting a dictionary I ask people to use it in a sentence even if they use it wrong so I can fit in instead of being right!") for it to still be what it is.

One word can mean different things in different contexts or among different groups of speakers. I'm pretty sure that you will find many of the people who use "vagina" broadly most of the time differentiating between "vagina" (now in the narrow sense) and "vulva" when such a distinction is necessary.

In my local German dialect "foot" can be used to refer to the entirety of the leg. That doesn't mean there's an inability to distinguish between foot and leg when the necessity arises.

It's a commonly misused word but it's definition hasn't changed because it's used correctly just commonly enough

Same question I asked Johava in the comment she screenshotted: Then would get to change the definition? The makers of dictionaries? (If so, nothing stops me from making a dictionary containing the meaning I favor.)

As long as there is a word for vulva, vagina's technical definition will remain the same

Not a perfect example as we don't know whether there was a commonly accepted term for the external female genitalia but in classical Latin vulva is the uterus.