Why doesn't the water evaporate in my dryer vent?

Well keep in mind that your clothes are wet when you first put them in the dryer, the hot air and the tumbling forces moisture out of the dryer through your vent. If it's walls are coated with lint like most are with continual use, the lint stays wet, moist and will eventually dry if the dryer if not used for awhile (which rarely happens as you got to have clean clothes). Some vents (but very few) have no access to the outside and must be vented inside through a mechanism attached just outside the dryer and which uses water. - you have to clean the lint from this type of vent on a regular basis and they are so rare that I doubt you have this kind of venting system - most just vent to the outside of the house. Check your vent opening on the outside of the house, assure that the "flap" is opening when the dryer is on and forcing hot air out. Also you might check the inside of the vent by taking it off, inspecting the length inside to make sure it's not blocked or clogged with lint. Or just replace it with a new one. Hope this helps.