Given the pattern of population distribution, I suspect that the Volkstaat would begin as a series of isolated enclaves. Once it became clear that using convoys to connect enclaves to each other was both expensive (in terms of fuel) and dangerous (because of IEDs and mines), the Kommandos (as the formations of the Volkstaat would inevitably be called) would conduct operations to both connect proximate enclaves and evacuate those that, because of size or location, proved too hard to defend.
If the leaders of the African National Congress were wise, they would respond to these operations by filming lots of footage of the refugees that would inevitably result. This might well trigger the sort of international intervention that, in our own time line, took place in Bosnia. However, it is also possible that hotter heads within the leadership of the African National Congress would prevail, thereby precipitating a great deal of violence against persons of European, Indian, or mixed descent living in areas not controlled by the Volkstaat. That, in turn, would cause many persons to either leave the country, join the Volkstaat, or create enclaves of their own. (One can imagine, for example, the creation of a state, centered on the city of Durban, that was dominated by South Africans of Indian descent.)