Is water vapour on light switch a fire hazard?
Water/ alcohol would be more of an electrocution or shock issue than a fire issue. 
Alcohol is flammable yes but depending on the concentration there’s probably more water, and water with impurities can make a conductor and shock you with contact being made on only 1 conductor , so I don’t see it as a possible fire hazard but as a possible shock hazard in reality. I spray any cleaner / sanitizer on a wipe then clean the device as the electronics are not sealed and it is possible to get enough moisture in the switch to get shocked.