What is/are the approved method(s) for draining the condensate line ? My furnace is located in the attic and I would like to drain into the house waste line by attaching the condensate drain line to a vent pipe in the attic. There is already a p-trap at the furnace.

I believe that that is an accepted practice. Not a heating man but from what I understand, if the trap of high efficiency furnace freezes, the furnace will not function. Put trap below insulation is not already so.

Are you certain that your firnace drain has a trap, most do not since they gravity drain to a floor drain. I'd check with the manufacturer to be certain, then you can tap into the vent stack but it must be a water tight seal.