Let me explain something clearly (I hope). Almost all organic liquids are flammable in gaseous or vapour form. The point at which this takes place is the flash point.

However, if we have any volatile material form through heating, even if it is below the stated flash point, it still can diffuse through a room. Now, this vapour, if it contacts an open heating element that is at or above the flash point, the vapour will ignite. These vapours tend to hug the ground or lower levels due to their density.

If you are able to gain a sufficient concentration in air, you get either a fire, or an explosion. It takes about a 50:50 mixture of commercial gas or gasoline fumes in air to explode (IIRC), but otherwise you have a large fire. Depending on the quantity of flammable vapour involved, the fire can be small or large.

A small amount of liquid can convert into a large amount of vapour. AAMOF, 1 mole of liquid can convert to 22.5 cubic liters of vapour at room temperature(correct this if I've disremembered, but I think that is the correct conversion). At higher temps, this volume is larger by the equation PV = nrT.

So, if this is correct, then 18 grams of water will make 22.5 cubic liters of gas (at room temp). PG will do much the same from one mole of PG liquid.

So, be careful when heating any organic solvent that has the potential of being flammable. Any open flame at or above the flash point in the same room can cause a flash fire if you form vapours.