I am not sure that turning off the hot water heater as well as the furnace for a run when the temp outside is only 25F would go over well with the family, especially since my wife is always cold when the thermostat is set at 72F. I have my furnace room right next to my stillin room. Most likely what I will end up doing is having two fans, one for each room blowing away from any appliances.
If we are going to talk about what could ignite ethanol vapors, then we shouldn't use light switches, tv's, fans, or even a radio, while stilling.
I think the key to this hobby indoors is ensuring no leaks, have decent ventilation, and don't leave your still unattended.
As for escaping vapor just condensing in the air, I believe that the relative humidity level in the home would have to be high enough for the vapor not to remain vapor since the atmosphere, inside or out, can hold a certain amount of liquit without it condensing - if that makes sense. Think of it this way, if you boil a pot of water on your stove to the point where it all boils off/evaporates, it doesn't start raining in your kitchen. The air can hold the liquid in vapor form.
Overall, turning off the furnace and hot water heater is something to think about as well as having a fan at the entrance to the furnace room pushing air/vapor away.