I’m a little confused… what is this PVC vent pipe?
For the plumbing vent and a furnace exhaust gasses vent it is normal for condensation to appear on interior walls of the pipe. Simply because there’s a water vapor in sewer gases and high efficiency furnaces exhaust. Also, if those pipes are properly installed / pitched (no dips, sags) there should be no condensation accumulating – everything should flow back into the sewer / furnace.
Power vented water heater doesn’t normally produce condensation but in a cold attic environment it may happen. Again, it should flow back to the water heater.
Dripping inside wall; might be another improperly installed pipe that holds water and any excess of water drips over the edge. It could be some leaking water pipe as well; maybe it drips into some area that prevents detection… difficult to say without further “investigation”.