Cats are not that smart with what they eat sometimes. Take anti freeze for instance. The ethylene glycol in it is poisonous to them and there have been numerous occurrences of cats being poisoned by this and they actually appear to like the taste of antifreeze. So the argument that cats know what's best for them to eat is misinformed.
Propylene Glycol also is toxic to cats. It is not really an allergy, more like a species specific toxicity. It is believed they can become anemic from it because the PG causes their red blood cells to have a shorter life.
Not to mention the obvious poison - Nicotine, which apparently could cause toxicity if as little as 2 to 4 cigarettes were ingested by a 10lb cat. So, one would assume it would not take a lot of
juice to pose problems.
Bottom line, your
juice is probably worse for your cat than any other pet you may have.