Boilng vodka for A/C cleaning
Alcohol vapors can certainly form explosive mixtures with air, so filling the car with ethanol vapors is potentially dangerous. A source of ignition, for example a spark, could under the right circumstances ignite the vapors. If the fan motor of the AC system is a brushed motor it can create small sparks. So I would say there is a fire hazard. 
But there are other problems with your suggested setup. The ethanol and water vapors would condense on every surface of your car's interior, the windows, the roof, the doors, etc. This would mean you loose a lot of alcohol, so you would need more. Ethanol might attack and start to dissolve some of the interior, glued parts may become loose. And of course the car would smell very strongly like vodka for a long time.
So I would suggest disassembling the dash cluster and whatever else you need to get to the AC system, and using the vodka in liquid form to clean it properly.