Hanoi is divided into two main factions: the South First party led by Le Duan and the North First faction, represented by Ho Chi Minh. Le Duan controlled the totalitarian state, so his hold on power was pretty secure from military attrition in the southern war, but it might be possible that focused and conscious U.S. efforts could disrupt this control and empower the opposing faction.
Failing that, South Vietnam needs to be made indefinitely safe from conventional invasion. The best way to do this is to extend the defensive line in the north from Dong Ha on the sea to Savannakhet on the Mekong in Laos; this would greatly shorten the length of front the allies had to defend and patrol against infiltration (170 miles vs 680), allowing them to economize on manpower. With a greater density of fortified positions to defend, they would also hopefully take fewer casualties. This would be the duty of allied forces (American, Korean, Thai) as the ARVN conduct pacification of the interior.
There's no way the NVA are running truck relays and oil pipelines through a cordon of ~8 divisions, so the insurgency in the South would be gravely weakened. There, you can make ARVN battalions organic to problem districts, as well as pursuing something like the strategic hamlet initiative. Additionally, the problems of getting NVA into Cambodia would probably make it easier to pressure the neutralist government there to stop allowing the Communists to use the country as a base. As more and more of the country is pacified, this would free up ARVN divisions to take up places in the northern cordon and shift out Allied troops.
Their left wing can be secured by beefing up the Thai army to about a dozen divisions; this would call for something like .75 bn$ a year in aid from the US (after the initial costs of equipment and raising formations), and only about half the annual classes of prime manpower. In the case of a conventional offensive by the NVA, they would be outnumbered by the combined allied force on exterior lines.
If the NVA offensive has its schwerpunkt west of the Annamite range directed against RVN, the Thai's can advance up to the DRV passes and cut their communications. If it's directed against the Thais themselves, the ARVN can operate against their flanks. With the Airborne and Marine divisions in the strategic reserve, they can envelop the NVA positions north of the DMZ; putting together an ARVN armored division at some point would make this easier too. If it's east of the Annamites going straight across the DMZ, the open terrain is probably going to leave them open to just getting blasted with superior firepower, while the threat to their flanks is still going to force the NVA to detach forces from the actual offensive.
