That vent on the side of my house ended up being a minor pain in the ass to fix. The vent is connected by a 4" pipe to the exhaust fan of my downstairs half-bath. It's on the side, about ten feet off of the ground. After eight years, the three flaps had in some combination disintegrated or been pecked away by birds. And the side of the house was splattered with bird shit.
The replacement vent I got from Home Depot was a wild card, and I wasn't sure how it would end up fitting. The shitty contractors who did the siding built this ridiculous frame around the old one. The pipe terminated flush with the wood frame of the house, and the gaps were filled in with that super hard foam stuff. The old vent seemed to be stapled to the inside of the pipe. The new vent was intended to fit around the outside of the pipe.
You can see how that causes a problem. Diana and I took turns breaking off pieces of the decaying plastic (seriously, was this thing made of corn?) until it was gone. While looking for screws, two birds decided to bring nesting materials to the thing. And mind you, the fan from the bathroom is on and pushing air out of it! The lip of the new vent was fortunately not very deep, so I kind of wedged it between the edges of the siding frame, and banged in some wood screws on the corners. The top two seem to have taken into the wood frame, while the bottom two seemed to really take to the hard foam stuff.
I have lingering concern that water will somehow land on top of it and go somewhere not desirable, but I think the siding frame should divert most of it away. Heck, before today it was a totally open four inch hole in the side of my house, which wasn't much better.
I feel accomplished.