Most standard vent flaps that I've seen have two thumb locks. They are VERY difficult to get locked or unlocked, and you have to push on them pretty hard to get them to lock. If you find them, use your thumbs to push up and in on these two tabs, and eventually it will snap into place. Reverse the process to get it back open.
FYI, you will feel like you are going to break it, but I have yet to break the one on my trailer or on my FIL's MH.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!