According to Dr Falkinham, cool mist vaporizers should never be used - even with distilled water. The only same kind is a steam vaporizer. Even then, the tank should be according to manufacturer instructions at least weekly.

As for bath vs shower, there is probably less steam associated with a bath, if it is warm water and not hot water producing clouds of steam - again it is about exposure - the less steam, you are exposed to, the fewer number of organisms you will breathe in.

For myself, I prefer a quick, hot shower with the bath fan running (and in summer the window wide open) to a tepid bath.