it is important to consider that the US invovlement in the conflict had three phases:
I. 1964-66 Conventional: VC/ NVA regular army forces tried to defeat US and ARVN forces in large force engagements but fail and suffer large losses in doing so.
II. 1966-68 VC forces conducted large scale insurgent warfare in an attempt to attrit US forces and destroy the socio-political structure of South Vietnam. THe situation became stalemated as the US could not end the supply by invading North Vietnam or expand the war into Cambodia or Laos as it would have destabilized the weak governments there and bring about a full scale North Vietnamese intervention in those areas.
The VC risked its whole organization and ended up destroying itself as an insurgent force after Tet. Afterwards, with no indigenous Southern support, the North resorted to interventions by conventional forces. This also came as Nixon opened relations with China as China joined the US in counterbalancing Soviet influence in Asia as the Soviet Union sought to negotiate better economic and military agreements with the US to limit the burden and tensions of the arms race on its economy.
Nixon seized the opportunity created by international conditions and eliminated the sanctuaries in Cambodia and the South Vietnamese army invaded Laos and severly degraded the NVA positions there.
The North, recognizign the situation, attempted to gamble again by sending its entire army on an invasion of the South in 1972, which was not only stopped, but entirely destroyed by a very capable ARVn and US airpower. Nixon cut off the last remaining supply route by mining Haiphong harbor.
The North realized that it had to get the US out of the south to have a chance at conquering the South. nixon's strategy was to guarantee the South's survival by supplying it and placing US air and seapower in position to support if ever threatened again.
Of course, Watergate changed the political context of the situation. The radical McGovernite wing of the Democratic party assumed dominance in Congress and literally overturned the landslide victory of Nixon's relection by reducing and then stopping all aid to the South as promised by Nixon, the prestige of the US to an ally and guaranteed by 50,000 US lives.
The South fell after its armies, out of fuel, short on ammunition and spare parts (with the world's 6th largest airforce) and US aid and assistance neutralized by Congress was abandoned and left destitute.
The congressional legacy of that time where we snatched defeat from the jaws of victory lives on today with Senator Joseph Biden, who sponsroed much of the legislation that cut off aid to the South before and during the invasion. I am not usually passionate about politics, however, Biden to this day earns my scorn and disdain.
So,, invading the North in the end was not neccesary. US and allied firepower had devasted the North's armies and its supply sanctuaries were eliminated as its patrons were competing with each other for better relations with the US. Johnson produced stalemate that saved the South, but Nixon had given it almost a sure guarantee of survival.