Where the plumbing vent stack goes through the roof sheathing, without an overly large hole cut through the roof sheathing, the only flashing/sealing method required is the vent stack flashing where the shingles are.
The best vent stack flashings are the lead ones, commonly referred to as "lead boots', with another approved type being the neoprene rubber types which slip down over the vent stack - the problem with the neoprene rubber types is that they either not UV resistant, in not sufficiently UV resistant and fail due to sunlight deteriorating them and cracking them.
At the bottom end of the plumbing vent stack in the attic, where the vent comes up through a top plate or chase, yes, that definitely needs to be sealed for fireblocking and draftstopping under the building code, and for air infiltration and exfiltration under the energy code.
Was not sure which you were referencing so I included both answers.
Jerry Peck - CodeMan
Construction and Litigation Consultant
Construction Litigation Consultants, LLC.