Declare Peace and Victory, pull all US military forces out, stop all monetary support of the South, and leave the two sides to sort it out themselves.
We did this to South Vietnam. Why not save billions in scarce FedGov funds and apply the same policy to the Korean problem?
Today, Vietnam is a trading partner of the USA, not an enemy. Shouldn't we assume that a Unified Korean peninsula could also be a friendly trading parter of the USA?
Do not mention any "treaty obligations" we might have to South Korea. The USA had "treaty obligations" to defend South Vietnam militarily, should North Vietnam violate the 1973 Paris Peace Accords.
North Vietnam violated that agreement by overrunning South Vietnam with overwhelming conventional forces -- tanks and arms made in China and the Soviet Union.
The US Congress responded to the "treaty obligation" not by honoring the treaty, but, by passing legislation in late 1974 that prohibited funding of any military assistance to South Vietnam.
Why not save the money and abandon South Korea to its fate? I'm sure that they will be fine.