I already have an MFC gui standalone program. What should be done to make it a VST 2.x plugin? (It would a lot of rework if I use VSTGUI/win32/qt/etc - or is it possible/appropriate to use VSTGUI?)
Which VST interfaces (gui and others) should I implement for VST 2.x gui plugin?
You are worried about the GUI of a VST when in fact you should be worried about the structure of the rest of your code. VST 2.x hands you a HWND for a frame, all you have to do is create a child window that hosts your GUI. MFC, raw WIN32 - does not matter.
However, the real 'problem' is in the rest of the VST 2.x interface. You should study this interface and learn how it works. Then you'll be able to assess if your code is in the correct structure to easily interface as a VST plugin.