Thanks for interesting observation. Something I do not understand and never thought about before.
Boiling point of VG: 554°F (290°C)
But:
When glycerol (also called glycerin) is heated to 280 °C, it decomposes into acrolein
Acrolein - Wikipedia, the free encyclopedia
It is something I do not get: VG is decomposing befor it can be boiled? We need a chemist for explanation.
Good news is in normal
vaping conditions we do not heat VG to boiling temperature because
juice will boil somewhere in between boiling points VG and PG (or water, it it was thinned with water). Boiling temperature of PG is 370.8°F (188.2°C)