Just replaced my range vent as I had the same problem.
Age makes the hinge pin brittle.
I ordered an "exact replacement" but the holes were different.
Rather than fuss with it and still have hinge pin issues 2 or 3 years from now, I returned it and got one without a flap. Still had different holes, but I backed it with nylon screening (for bugs) and installed it anyway.
I used a razor scraper and carefully cleaned the old caulk off. Yes it was a pain, but the only scratch I made is hidden by the vent flange. I used Proflex to caulk the new one in.
Oh, I did need to modify the replacement (and would have needed to do the same with the "exact replacement") as they are made for a deeper wall than mine. My wall was thinner and so I had to saw an inch or so and open the hole a bit (it was not square).
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW