Should I insulate a metal pipe roof vent to stop condensation?
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:


Go into your attic and cut it off below the roof line. Repair the roof sheathing and roofing. 
Replace the flashing from above. 


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.