I ran across this tid bit of info which leads me to believe that if VG were to become toxic while using it in an ecig, you would know it.
Acrolein, has a pungent, suffocating odor, is toxic by all exposure routes. Exposure causes inflammation and irritation of the skin, respiratory tract, and mucous membranes. Inhaled, acrolein is irritating to the upper respiratory tract even at low concentrations. Its odor threshold of 0.16 ppm is similar to the OSHA permissible exposure limit (0.1 ppm); thus odor may provide an adequate warning of potentially hazardous concentrations. Inhalation may also cause an asthmatic reaction in sensitized individuals. When ingested, acrolein produces chemical burns of the lips, mouth, throat, esophagus, and stomach. Nausea, vomiting, and diarrhea also occur. Apart from rare cases of sensitization, no adverse effects in humans chronically exposed to low concentrations of acrolein have been reported.