If Nixon is less stupid in handling Watergate, AND if he decides to actually deploy the National guard divisions, then a failed peace negotiation might see a major redeployment. In order for the war to last till the 80s, the Americans need to feel that they have won and simply need to outlast some rebels.
Have the Guard units trainseparately, providing a different army to the one worn down in Vietnam, and then launch a general offensive into North Vietnam from the South, and a double sided amphibious invasion of Hanoi. Make it clear that you aren't interested in co querying the whole country, but force the main MVA units onto major combat. Following this, have the Americans essentially take over the South Vietnamese government MacArthur and Japan style . Have "old army" task groups launch assaultsnon known NVA/VC positions in SV, while having the Guard or "fresh army" units secure south Vietnam in sections. First secure the Capital areas, then secure the Mekong Delta, the islands, the areas south of the Mekong Delta. Have mobile battalions specifically designed to attack into surrounding countries to cut off NVA supply lines.
Give it a couple years, and expect heavy combat at first, but with Hanoi taken the American populace finally feel that they've won. After this you can transition to more local forces.
The reason that this pushes the war on so long is that there are still two hostile foreign powers (USSR, PRC) combining with the remnants of the NVA to wage war, creating a place for the insurgency to be promoted and capital to fund it.