How, if at all, do you plan to represent the instability, and constant forced change of, the South Vietnamese regime?
Without going into too much detail, there are two ways in which I plan in replicating the instability.
The first is through the use of the 'Republic' government. Thanks to TNO's 'Stack Kills' feature I will no longer need to add the 'fortress ' feature to all the map tiles, which means that all SVN units not in cities or on fortresses will generate unhappiness and thereby increase the risk of cities 'revolting' (I may either forbid the building of fortress tiles or strictly limit the number of engineers units they possess during the game to limit the number of fortress tiles they can build).
This in turn may reduce the amount of economic aid they can receive per turn, which is crucial for buying all their non-infantry related weapons systems, i.e. tanks, artillery, planes, etc (because their cities will have very limited productive capacity, it will be much quicker to buy these units then to build them, therefore the more money they have the more they can buy). The generation of the SVN infantry units will be event driven so they won't need to build them but they will be homed to SVN cities, which means there will be a limit on the overall number of units they can support based on their overall productive capacity.
Because the period of instability was more prevalent at the beginning of the conflict I will allow SVN to research the CORDS (Civil Operations and Revolutionary Development Support) technology, which will lead to the RUFF/PUFF, Pacification and Phoenix Program advances. The RUFF/PUFF advance will allow the player to build the Regional Forces (Police Station) city improvement and allow them to start receiving the Popular Forces unit through the events. The Police Station will obviously bring more stability to the South's cities by allowing more units to operate out of cities. The Pacification and Phoenix programs will lead to the South's fight to take back the countryside from the NLF thereby reducing the enemies ability to recruit new VC units.
The second method I also envision is creating random coup events which when they occur removes the tech that allows the SVN army to recruit their new infantry units. After a few turns they get the tech back.
Those are the current broad lines but as usual only testing will determine how well my concepts work.