The photo you have posted seems a little odd. Was that done at the factory? if you look at the Scamp
and Casita web sites and the photos you will see none of the vents stick out like that or not that I can see. I have a water heater and fridge
vents on the outside of my scamp
and they dont stick out like that and I dont think there is anything special about them. The bottom half of fridge
vent is mounted fairly low on the trailer and its flush to the trailer - the water heater vent is a bit higher (top of it about 6" down from belly band) and the bottom of it is not totally flush but the bottom only sits out from the trailer less than 1/2". I think you will find that most of the vents are thin enough that they flex a fair bit for mounting.
Looking at your vent I suspect it was mounted it a bit too low. Vents that are mounted that low on the curve part of the trailer are usually not so large so that they can be flush. You may have to try to find a two part vent with a hinge in the middle in order to get it to sit completely flush due to the size.