c++ /vb.net custom dll - import signature definition problem

I am trying to re create a VB.NET forms application that reads an existing tiff tag contained inside a tiff file that was created by an older program.(we no longer have access to the original program code - only a few fragments of code) The data created for the tag is compressed and decompressed by an old dll written in c++ about 20 odd years ago. The original program that used this dll was written in VB6. I can reference this dll in my new VB.Net program using dllimport - and have managed to find the entry points and calling conventions and almost got the program working apart from the issue i have with the managed PInvoke signature not matching the unmanaged target signature. I'm probably wrong, but looking at the description of the variant, it looks like it's a byte array that contains two single point values and then another byte array that contains more byte arrays that contain further information. I'm only interested in extracting the two single point values from the tag however from what i have learned looking though some of the other guides and suggestions online is that this signature probably needs to be a structure? I have the code that created the original dll in c++, so i'm hoping that this should be sufficient for a c++ expert to quickly identify what the calling signature should be. As mentioned i'm only interested in decompressing the tag. (The DataUncompress Function) c++ code attached in a text file along with a description from the original program. I also have a some sample tiff images that contain the tag with the compressed byte data so that the proposed solution can be tested. If you require more information prior to quoting then please let me know. Thanks for your time.