Acidic, yes. PG is broken down into pyruvic acid (glucose metabolism), acetic acid (ethanol metabolism), lactic acid (digestive) and propanal (possible ethylene precursor) How much of an impact getting vaporized amounts of (say) 10 ml of PG into the lungs each day would have, that would seem to be negligible. At least compared to body weight, drinking a single 360 ml cola or eating a 100 grams of chips.