2. Um, I think that's it. But isn't that enough?
Update: Done. And ugh. The attic work was in some ways the worst part, but the joint is nice and tight, and I'm not worried about leaking conditioned air into the non-conditioned attic.
On the other hand, removing the vent left a good-sized gap in the insulation that I tried to fill with Great Stuff, and that might not have been the best choice. For one thing, I have a jaundiced looking swath of yellow on the back of my right hand that may well be there for a couple of weeks until I get enough new skin for the gunk to flake off. Man, that shit is sticky.