Duct tape is best suited for flat surfaces that are not exposed to heat or weather elements. It is also not suited for weight-bearing repairs. When exposed to heat, duct tape will become soft and anything it is holding will sag, including vent pipes. Over a long period, duct tape will also dry-rot and leave nothing but threads.
One type of duct tape, known as foil-faced tape, is designed specifically for vent pipes. Foil tape will not fail and it provides a tight seal between pipe sections or seams. This tape is available at heating and cooling stores or at most home remodeling centers that sell vent pipes.
If your vent pipe also has fittings such as elbows that allow your pipe to be situated where it needs to go, use foil-faced duct tape and clamps. Vent pipe clamps will keep your fittings from sliding off your pipes, and the tape will provide added security.
To apply foil-faced duct tape to your vent pipes, simply stick the end of the roll to the pipe and start winding tightly around the pipe, or cut three 7-inch strips to wrap around the pipe. It should not take any more tape than that to provide a good seal.