When it comes to "Insecticides", they allow the use of carcinogenic, poisonous, toxic, and volatile chemicals for processing. (They can also be present in the finished product.)
This becomes the issue of manufacturing...
Why would you not just use tobacco extract yourself? Even if you didn't remove all the other junk, you are still not burning it. (NOTE: It only takes 1/10 of a cigarette to make a cigarettes worth of nicotine juice. One pack makes two cartons worth of nicotine juice. Based off 20mg per actual cig, and 2mg intake per actual cig.)
Even if the FDA regulated nicotine directly... It would still be available as "Food grade", as "Food grade", is essentially anything that can be consumed. (Not that it is healthy or good or contaminant free. It just has to have levels of contamination that the FDA finds allowable. Such as a carton of orange-juice can have 20 fly-eggs and up to 2 maggots present, and that is FDA approved for consumable food, or "Food safe" production.)
The only real grade of production that is worthy of this level of manufacture, is "Pharmaceutical grade", and possibly as it is now, using "Food grade" as a second best choice. (If it is not allowed inside a human, it is some other grade. No grade just means that the manufacture has not submitted it for a grade. It could be food-grade or pharmaceutical-grade, or better, or worse. You won't know until you test it, and compare it to one of the grades.)
Final note...
It is not possible to safely extract pure nicotine at home. Tobacco-juice, or nicotine-sludge, is not pure, and will contain many harmful chemicals. It would not be wise to invest in trial and error, when playing with unknown volumes of poisons that you can not measure or confirm as pure.
This world is heading for a big change... I'll be living on the moon, watching from a distance... because that is where I am sending my ashes...