I can't imagine it would be any worse than smoking around a pet, as far as nicotine goes. We don't smoke inside of our house now, but at our old house we always did. My parents smoke in their house as well. My cat buddy had lived in a smoking house his entire life and was 21 when he passed away in Sept 2007.
I was curious about the PG around pets, specifically cats. There isn't much info out there on it, and I found this on wikipedia:
Animals
Veterinary data indicates that propylene glycol is toxic to dogs with a 50% chance of being lethal at doses of 9mL/kg, although the figure is higher for most laboratory animals (LD50 at levels of 20mL/kg).[14]
However, propylene glycol may be toxic to cats in ways not seen in other animals. The U.S. Food and Drug Administration has determined that its presence in or on cat food has not been shown by adequate scientific data to be safe for use. Any such use is considered an adulteration of the cat food and a violation of the Federal Food, Drug, and Cosmetic Act.[15]
I'm very careful to keep the juices in a sealed box so my guys don't get to them, I don't know how much of the exhaled vapor is water and how much is PG.