I "squared off" the hole with tin snips. I then cleaned off all the old Vulcum by scraping as much as I could off with a putty knife, then using a "Scotch Brite" pad with mineral spirits. The pad DOES scratch the aluminum, but all of that is under the flange & not visible after installation. Underneath the original rivets, on the inside of the outer skin, was a layer of white putty, used to seal the rivets. I just forced a bit of that putty into all of the remaining original rivet holes. I got lucky in that none of the vent's screw holes coincided with any of the original rivet holes.