I am not sure of your question. I understand the vents do not fit the hole in the siding. How much gap between the sheeting and the vent assembly?

As I understand the vents are in the side walls, not in the gable. Are they under the eaves, or down lower? Are they in a position that rain can come in them?

I have a 10 X12 shed with two vents in the very peak of the gable ends. I have never had any problem with rodents.

The vents will not keep the shed cool. Since there will be differences in the heating of the two sides of the shed, the temperature difference should pull the air through the shed.

According to the installers supervisor - the vents are to be installed high and low. When I talk with Yardline I will verify if he was telling me the truth. Here are pictures

It looks like the vents are not quite installed properly, they should be flush with the siding. However it LOOKS like there is nothing wrong with the vent that a little fussing and some caulk would fix. If you are still concerned you could frame the vent on the inside with some 2 X 1/4 lath or some screen door molding. Again with the proper caulking. This would give a more finished look inside of the shed. You may wish to add, some half inch opening wire mesh to the inside to keep the mesh from getting damaged.

The placement of the vents is the way we ventilate our house. On nights it is warm, but not warm enough for the AC, we open the upstairs windows on one side of the house and the downstairs windows on the other side of the house. The convection current created by the air raising in side of the house brings sufficient air inside to cool the house.

I have considered adding the lower vent for the above reason and so they are in the position of your vents.