Alright, I'm SO CLOSE, but I still have a bit of a problem.
So I tried the 32/64 bit thing. I tried changing it in the Build->Configuration Manager menu, both the Platform and the Active solution platform. It's odd, because in both cases, the only option initially was Any CPU, but I clicked New and there were options for Win32 and 64 bit, but no way to customize their options (other than copying settings from an existing one, which I also tried). Why aren't there any options to set, anyway? Is it sufficient to just pick which one you want and leave it at that? And if so, why do I have to choose them from "New"; shouldn't they just already be on the list?
Anyway, the third party DLL I'm using (DLL A) is apparently available in 32 or 64 bit, so I tried both. I tried them directly in the project, setting the platform settings to match, and I alternatively tried putting DLL A into a C++ project to make my DLL (DLL B), and then import that one. In that case, I had to make everything 32 bit, because it seemed like my C++ project was stuck that way, but I was able to compile and import that into my other project.
In all cases, when I called the functions, I got DllNotFoundException or BadImageFormatException, and usually I could manipulate the first one into the second one by changing some settings (it seems to me that the second exception is not as bad, and closer to the goal). But no matter what I did, I couldn't get rid of the exceptions completely so I could actually run the functions.
Also, when I "import" DLLs into my project, what I've been doing is placing the DLL, LIB, PDB and another file (I don't remember the extension off hand) into every folder within my project JUST IN CASE it's looking for it somewhere I wouldn't expect, then inside the project I "Add existing item" and pull them all in that way.
I thought that would be sufficient, but then I noticed that there is an "Add reference" on the menu, which then allows me to choose DLLs from a list of ones that are apparently already associated with the project in some way or another (like mine because I included it in the project). On this list, I checked the box to import it and clicked OK, but I got an error. Unfortunately I don't have the text with me, because I wrote down so many notes about errors I accidentally brought the wrong one, but I think it essentially said that it couldn't import it for some reason or wasn't compatible or something. Sorry if that's vague. I'll try to be more specific tomorrow when I bring the note.
So if anyone happens to know what I'm doing wrong, I would appreciate if you tell me please! Thank you.