What I know is that at the base, it is toxic, but still used in the respiratory tract.
Water is toxic if you drink enough of it.
The toxicity is in the dosage. A fatal dose is considered a MINIMUM of 50mg per 1kg of weight, so if you weighed 180lbs that would be 81.6kg, which would mean you would have to consume at LEAST 4kg (8.8lbs) of pure menthol to die. The amounts you're going to run into while
vaping is less than you would get from smearing Vicks Vapor Rub on your chest if you had a cold.
Menthol itself is usually extracted from peppermint.
To the OP...
Can you not order menthol crystals from the internet? They're cheap, usually around $5 for 20 grams which is enough to make 30ml of strong menthol concentrate. It's commonly sold for aromatherapy use and I'm not aware of anyplace that has bans on it.