Propylene glycol and glycerol (vegetable glycerin) are both sugar alcohols, as is ethanol. You're not vaping alcohol, though. You're not vaping alcohol, silly, you're vaping the steam/vapor/whatever that is created when the alcohol ignites. I can't think of any alcohol that wouldn't burn at these temperatures. What makes glycerol and propylene glycol so good for vaping is their viscosity and low volatility.