Avoiding the million or so sent to "re-education" camps, the hundred-thousand or so who were outright executed, hordes of desperate people trying to flee in leaky boats, the economic chaos and near-starvation conditions of the late 70s due to forced collectivization.
The pattern we've observed...
The only thing off the table in this scenario is American ground troops. And even there, we could fudge a little bit...maybe some very limited, almost certainly secret, involvement by some special forces troops. Something like this for example...
Assume this policy of "no ground troops" is informal. Nixon doesn't want the North to know it, but he's much too canny to risk the political backlash from such a move.
How long can S. Vietnam hold on? I would guess, at least through the end of the Nixon administration, and that after that it...