I am not a biochemistry student so honeybadger can step in if I get anything in this wrong. I do have severe perfume allergies and this is what the doctors have told me. Most modern perfumes use scents derived from fossil fuel derivatives like coal tar and such. They are supposed to be volatile (they evaporate) because if they weren't they would have no odor and scentless perfumes are an oxymoron. They can't be too volatile because then the scent wouldn't linger and the wearer would lose his or her allure too soon. Eventually these aromatic hydrocarbons will go away if allowed to vaporize into the air whether on skin or clothing or celluloid.
I bought a Sheaffer's Balance a few years ago. It was a great pen, a size and color I had been seeking for years. It also reeked of Obsession which is one of those scents that can put me in the emergency room. It spent six weeks on a pedestal made of old film canisters on my screened in back porch before I could bring it back in the house again. If you really like your Vac you may have to do something similar. The key is air flow over time.
Good Luck. Vacs are nice pens well worth saving.
You got things mostly right. Odors can cling to objects longer than human skin, but like you said, several weeks can eliminate even the most intense odors because in order for you to smell anything, they have to have a high enough vapor pressure to evaporate, which means the odor causing compounds are leaving the pen. Stagnant air is what keeps them on the pen, you need lots of airflow.