Based on your comment I think what you will need to do is replace the insulation.
As I said in my comment condensation is caused when warm moist air hits a cold surface, be it a window or roof. If your insulation was sound then this would prevent this happening, but it sounds like it letting the air through to the roof.
I'm reluctant to suggest what to replace it with as I haven't seen the roof - so it might be worth getting someone in to quote for the job and ask them what they intend to do.
If you do decide to replace then putting a semi-permeable membrane up as well will help keep the moisture off the cold surfaces.
Options include looking for discoloration on surrounding areas and even putting a paper towel or Kleenex in suspect areas in order to detect a small leak that may not have been sufficient to create noticeable discoloration. Depending on the volume, you may be able to hear dripping in order to help trace the leak. And you may wish to reconsider whether the initial diagnosis was correct or whether the repair was sufficient.
Here are three causes that weren't obvious to us that you may wish to consider:
(1) Around the tub spigot: We had a situation in which we had a leak from a bathroom on the ceiling of the dining room beneath it. We re-caulked the tub and shower surround, did a small test, didn't see any water leaking, and thought we'd taken care of the problem. The next time someone showered the leak reappeared. We eventually figured out that the problem was around the spigot: Water from the shower was hitting the spigot, and because there was not a good seal around the spigot, water was running behind the spigot, down to the floor, and eventually into the dining room below.
(2) The actual copper supply line connections--if you're fortunate enough to have access to them without tearing out the wall: In another house, one with only one floor, we thought we had a leak in our shower pan, but during a remodeling discovered that the hot supply line had not been soldered correctly and water had been squirting out between the wall and eventually out through the brick in the adjoining exterior wall.
(3) Diversion due to insulation: We had another leak that we thought was due to a problem with the flashing between the roof and adjoining siding. After tearing up the roof and the ceiling of the room being damaged, we discovered that the problem a poorly-sealed nail in the roof itself. However, this location was several feet away from where water damage showed up. In this case, the insulation directly below the small leak was sufficient to keep the water from running straight down. Instead it wound around and into another room.
Best Answer
If I understand correctly, you're saying that there is moisture getting past the seal on the old (metal) pipe's flashing, but not past the seal on the new PVC pipe. This is probably due to a failing seal, and not condensation. If the old pipe is abandoned, I'd do one of two things:
It's possible that you're actually seeing condensation if the attic has very poor ventilation and there's a source of moisture somewhere, such as a roof leak. In that case, fixing the roof leak should eliminate condensation.