I am a little confused: Are we talking about a plumbing vent, or the room exhaust ventilation? And in either case how is a 'vent' from a 2nd floor lav in the ceiling below?
NOW: If it is a room exhaust vent, I suspect that the condensation is on the inside of the pipe and leaking out at the joint.
If it is a plumbing drain pipe: condensation is caused by warm moist air touching something cold. Usually, there is not warm moist air inside the ceiling cavities.
Please help us with a further description of the problem.