Is there any way to edit diplomacy so that a vassal civilization has to comply with all of its overlord's demands? I know that vassals must provide their overlords with resources, it stands to reason that it's a short jump from that to expanding that category to include conversion, technology, gold, etc.

The best way to probably attempt this would be to edit the AI_techTrade() function in CvTeamAI.CPP, there actually appears to be a commented out line already in the function that if uncommented would do the trick:


/*
if (isVassal(eTeam))
{
return NO_DENIAL;
}
*/


There should be equivilant functions for conversion, but I was under the impression gold was handled the same as resources.

I haven't tried it yet myself, so I can't make any promises for its success. I am planning on doing some more diplomacy editing in my home brew mod, so if I end up changing that I'll post how it goes. :)

Please do check this out, these files get confusing for me as I'm not very good at programming. Where do I find this file, and how do I open a .cpp?

The game source code is in the main BTS folder in a folder called CvGameCoreDLL. A cpp contains only text, so technically any text editor (Notepad) can open it, but you'd want to get something which knows about C++ syntax. Once you make changes to these source files you'll then need to compile the source code to create a new DLL. Both MS Visual Studio Express and CodeBlocks can be used for this, hunt around for threads on getting them set up ... it's not trivial, but there are people who will help get you going.