Hi mama-
We have both in our house (and need to get at least one more of each now that the kiddies are in seperate rooms...).
A vaporizer is typically hot steam and a menthal vapor concentrate can be added. My family has always trusted Vixs and that is the brand I purchased. I use the generic vapor concentrate from CVS. We use it when the kid's have colds, congestion and stuffy noses. The warm moisture loosens things up more (in my opinion) and the vapors help tremendously. Things seem to drain easier. A few nights with this running and the kids are usually good to go. I also still use the rub directly on the chest - when they are old enough (last winter our son was about 3 months and too young for the rub). My only complaint is needing to change the water solution daily when in use. For some reason, the vaporizer turns the water "black". I think it is more esthetics than hygiene.
A humidifier puts moisture into the air too but I think the purpose is different. It prevents dehydration versus being medicinal in nature. The human body has an optimal moisture range in which it functions and a humidifier helps achive that in the air you breathe. IMO, I think the cool mists work best. It seems to be absorbed by the dry air faster. I also like the sonic units better than the ones with filters - the filters have been too much work! We have forced air and totally would not sleep without a humidifier (unless of course we are using the vaporizer since I use it too if needed...)
Hope this helps.
~C.