Also, PG has been used in ventilation systems as an air sanitizer for decades. Not a 1:1 comparison given the massively increased volume papers get, but it's something.
There's literature back to the 1930s about workers inhalation of VG and PG vapour in volume in an industrial setting. From memory, there are consequences but not associated with the chronic degenerative health conditions associated with cigarette smoking. An examination of the molecular biology of VG and PG suggests its quite difficult to find bad things they would do that would be associated with the chronic degenerative health conditions associated with smoking.