One of the nice things about the Dark GDK is that you could #include things from the C++ Standard Template Library because Visual Studio has them as, well, standard. (The STL is a multiplatform standard and comes with every standard C++ compiler on current platforms.) Will AppGameKit allow me to include STL libraries for a Windows app and still have the "bam, now it's on my iPhone!" without headaches?

I don't plan to do much complicated stuff, as I'm still working on learning CS1 level programming, but it would be nice if I could use vectors, for instance.

I think this explains the answer to your question, especially regarding STL usage on iOS.

However, I can't guarantee that AppGameKit itself will be compatible with the STL as DarkGDK(DBPro in C++ form) had to be set with specific compiler settings in order to use the STL.

From what I've read:

Yes, AppGameKit tier two will be fine, because that's c++ compiled natively. If AppGameKit has any object-oriented things... then maybe not. I remember there being some issues, for example if a class doesn't have an assignment operator, or certain methods, then I think you'd run into issues putting that into certain template classes.