I think foxglove is the next highest after tobacco. Like most members of the nightshade family, it's a real chemical factory producing compounds you wouldn't want to vape - or ingest.
I think Foxglove has Digitalis (sp?) in the Flowers and or Leaves.
BTW - Here is a Chart of some Fruits and Vegetables that contain Nicotin
Check out How many Grams you would need to Make 1 Microgram of Nicotine.
Gonna Need a Whole Lotta Eggplant to make 54mg of Nicotine. Even at 100% Extraction.
And 54mg of Nicotine is what a Person uses per day if they Vape 3ml of 18mg/ml in 24 Hours.