fixing lead boot on waste vent
From the photo you posted I can see you have a different problem from what you may think. The reason the vent pipe is damaged is because a "critter" has ben gnawing it away!  The most likely suspects in the animal world (in N.A.) would be in the rodent species: squirrel, rat, chipmunks, porcupines to name just a few. You can see the small serration grooves cut by their continually growing front and lower teeth.  What is extremely astounding is that the material is lead. A soft metal! You have a very determined and serious critter wanting 'in' to your house! Normally I would suggest en-circling the pipe with a wire mesh, but I would think it could rip through it and come back for seconds!    Repair or replace the vent as easiest and best you can. To stop the chewing apply a coating of roof patch to the pipe, covering it completely. I would try to catch the pest to prevent it from future damage elsewhere and from infiltrating your home.