I am going to guess that these are custom made soffit vents. Standard soffit vents are 6"x12", continuous vents, round vents, etc. I have never seen one the size that you specify. That, plus the fact that they are framed, leads me to believe that they are custom.
If they are, then there is no way to guess how they are installed, or how they are made. Your best bet would be to remove one, to see how it is made and installed, then make your decisions from there. Soffit vents are not usually too complicated, so it shouldn't be much of a problem. They are probably just frames with screen or hardware cloth stapled to it. Pull one off and see.