I have a wet spot in the middle of an interior wall. The area backs up to a bathroom and has a vent pipe in the cavity. The vent runs up to the attic then crosses the flooor for about 20 feet before venting outside.The pitch is away from the area getting damp.Any ideas?

Try wrapping the vent stack with several layers of toilet paper up in there attic but close to the floor. If you are getting any rain running past the boot the toilet paper will tell you. It's a start.