Here's another picture (click for larger image); it's not very well focused but you can see a little more. I don't know much about wasps or hornets or other flying things with stings, but I do know this: I don't want them in my attic. I especially don't want them colonizing other nice spots in the attic.
In that second photo you can see that the opening in the top of the vent is rather large. I'll say it's big enough for a small rat to climb through. Now the inside surface of the vent is rather slippery, and I wouldn't fancy trying to climb it myself, if I were a rat's size. But that wouldn't stop a mouse or rat from falling in and looking around. And finding some other places to build a nest.
I had some ½" wire mesh—that would keep mice and rats out. I also had some fine plastic screen; that would keep the insects out. I tracked those supplies down, and also some tin-snips for the wire mesh, an X-acto™ knife for the plastic screen, and a staple gun (with enough ½" staples).
Back in the attic, I held the wire mesh up to the hole to determine the shape and size to cut. The tin-snips worked reasonably well. Then, because I have lots and lots of plastic screen, I folded it over the wire mesh and cut it to size with the knife.
Now I had an assembly of two thicknesses of fine plastic screen around one of ½" wire mesh; it was also less hazardous to carry the assembly, because the sharp ends of the wire were covered. Fortunately there was enough room to squeeze into the spot below the vent. Then I started stapling.
You can see the result at right. If you look carefully you may notice that the hornet's nest is gone. When I drove the first staple in, the nest dropped down onto the screens.
I waited. No buzzing sounds. Or wings. Or stingers.
I reached around an unattached edge and gingerly picked up the abandoned (I hoped) nest. What to do with it? I fed it through one of the gaps in the vent. It fell through the opening and dropped onto the roof.
With that disposed of, I applied a half-dozen more staples.
Posted: Keep out! No trespassing!