[Ooper]

You ridge vent may or may not be defective but I doubt that it is. Ridge vents are designed to be very efficient, even in "sideways" rain. I would suggest that you have your air conditioning ductwork inspected as I suspect this is the cause of your water stain. Many times, if you ductwork or the insulation around your ductwork in not correct, the cold air will cause condensation to accumulate on the ductwork in the warm humid attic spaces in the summer time. This condensation will run down to the diffusers and can cause stains and other damage to the surrounding area. This happens frequently in bath and shower areas also.