Get rid of hornets: The nest has been removed, but the hornets keep rebuilding
I have had luck by spraying the nest with pesticide in the evening, when most should be at the nest rather than foraging, to maximize kill count and reduce danger to me because they are less active.

Then remove the nest and spray the entire nest area with the ubiquitous spray lubricant that comes in a blue and yellow can with the number forty on it. I do not know why this seems to deter them from rebuilding in the same spot, but have been told that it neutralizes the pheromone scent that attracts them to build in the same spot again. Try it on the vent pipe too.