The fact remains that in all the times I have heated glycol above its specified flashpoint, I have not had a fire that I did not set intentionally, and that was the one I mentioned in a previous post. There are other things to consider: does the vapor actually behave as you suppose? There are temperature gradients to consider. The container will be at a higher temperature than its contents during heating. The air-vapor mixture above the surface will not be at a higher temperature than the surface of the fluid from which it came. There will be a partial pressure of the vapor. It will be an indicator of flammability. The flashpoint is not the autoignition temperature. The autoignition temperature of propylene glycol is much higher. than the flashpoint. I have read the MSDS for PG, several times over and from different source. In addition, I have read the manufacturers literature about these solvents.

Aside from all that, I no longer recommend heating above the flashpoint and have not for quite some time.

Where is the hot element in a microwave oven? It is in fact the material that is being heated., and probably not at the surface that is in contact with air. In a microwave oven, heating to or even above the flashpoint will not be sufficient to ignite the glycol. Try it if you don't believe it. You will need an ignition temperature that is not found at any point in the oven.