Short answer:

The efficacy of strategic bombing in WW II was at best mixed as a return on the massive resources invested in bomber fleets: German industrial production peaked in late 1944. Since Vietnam didn't have substantial industrial capacity to begin with, there wasn't much to bomb in the first place. They weren't producing their own weapons, rather transporting weapons imported from China and the Soviet Union, industrial capacity that was untouched by US bombing.

Discussion:

In the 1920s and '30s, partly in response to the horrors of World War I, a new theory of strategic air power developed. The notion was that the capacity to bomb industrial capacity would limit wars, and avert the slaughter of World War 1. It didn't exactly work out that way.

The US Strategic Bombing Survey, a massive effort after the war determined that despite the huge resources applied to the bombing campaigns, the net effect on German industrial output was mixed. The bombing campaign against Japan did catastrophic damage to civilian populations, but again the damage to Japan's industrial output wasn't as promised. The submarine warfare campaign, which cut Japan off from raw materials was likely a more effective -- and less murderous-- strategic instrument.

When you look at the war in North Vietnam -- what was there to bomb? North Vietnam wasn't producing their own weapons, for the most part. Nixon made the decision to mine Haiphong harbor, one of the routes through which North Vietnam was being supplied. A very large bombing campaign attacked the Ho Chi Minh trail, by which route supplies were brought South-- but dropping bombs in a jungle to interdict groups of porters carrying supplies is a capital intensive solution to a human intensive problem. As a footnote, one can argue whether this was "strategic bombing" -- in the sense that it wasn't a campaign against a national productive capacity, but rather the interdiction of a supply line. That does beg the question: what would there be to "strategically bomb" in North Vietnam?

In 1972, the US did bomb Hanoi itself -- the celebrated "Christmas Bombing". Despite Henry Kissinger's claim that the North Vietnamese were "on their knees", it's unclear what impact it had beyond loss of civilian life. It may have pushed the Vietnamese to sign the Paris Peace Accord.

The PAVN (= "People's Army of Vietnam) 559th Transportation Troop had responsibility for moving supplies south on the Ho Chi Minh trail, and were skillful in deception and camouflage. A RAND study gives a picture of the scale of the resources that the North Vietnamese deployed to counter bombing:

Given its importance, it is not surprising that Hanoi would commit tremendous resources to constructing, maintaining, and defending the trail. At any given time, approximately 100,000 people were employed along the trail as drivers, mechanics, engineers, and porters and in ground security and anti-aircraft units. Anti-aircraft artillery appeared in 1965, and by 1970, the entire trail was protected by anti-aircraft guns, some equipped with radar. The PAVN’s employment of “hunter-killer” teams and tribal scouts also protected the trail against enemy incursions.

On can compare with Afghanistan, where US airpower has been dominant for two decades, and weapons of great precision have been employed, with little net effect. If there's no factory to bomb, strategic bombing is a chimera. Even when there _is_ a factory to bomb, as in Germany, most facilities are easily put back in operation. The most attractive target seems to be oil production and refining facilities- these were the German facilities that were most "productively" attacked, meaning that when hit, they were very difficult to rebuild, and German capacity was reduced.

Since Vietnam had little of that kind of infrastructure in the first place, such a campaign wasn't possible.

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Sources:

Clodfelter, Mark. "Beneficial Bombing: The Progressive Foundations of American Air Power, 1917-1945", University of Nebraska Press:2011

“U.S. AIR GROUND OPERATIONS AGAINST THE HO CHI MINH TRAIL, 1966–1972.” Special Operations Forces and Elusive Enemy Ground Targets: Lessons from Vietnam and the Persian Gulf War, by WILLIAM ROSENAU, 1st ed., RAND Corporation, Santa Monica, CA; Arlington, VA; Pittsburgh, PA, 2001, pp. 5–28.

Smith, Melden E. “The Strategic Bombing Debate: The Second World War and Vietnam.” Journal of Contemporary History, vol. 12, no. 1, 1977, pp. 175–191. ( a "revisionist" account-- making the case that strategic bombing is actually more effective than skeptics have it, read for balance)

Kenneth P. Werrell. "Death from the Heavens: A History of Strategic Bombing". Annapolis, MD: Naval Institute Press: 2009

Richard Overy: "The Bombing War: Europe 1939-1945", Allen Lane: 2013

So, this is because the Second World War and the Vietnam War were fundamentally different in that the former was a conventional, classic war while the latter is an example of unconventional, subversive war. The militaries of the Axis Powers had uniformed soldiers who, as such at least in theory, were bound by the Geneva Conventions to observe certain protocols during the execution of war. There was a clear command structure where orders are given and received by officers and enlisted men in carrying out varying forms of classic military strategy.

In Vietnam, however, the US military found it difficult to fight the North Vietnamese, the Viet Cong, and other groups because of these groups' reliance on guerrilla warfare, subversion, and other unconventional methods not primarily employed by conventional armies. Guerrillas found it easy to blend into the population, simply wearing their civilian clothes to try to avoid suspicion or capture. In the Second World War, targets of Allied bombing missions were strictly military targets, like bases, munitions factories, supply lines, etc with a varying degree of exceptions. In Vietnam, targets could not be identified as strictly military or civilian because of the nature of the war being fought. Therefore the US just bombed the country (as well as neighboring countries) almost blindly in an attempt to disrupt the reported flow of arms, funds, and soldiers, resulting in more bombs being dropped.

The Vietnamese were able to withstand such attacks because the war was mainly fought due to ideology, that being national liberation. Vietnam had been colonized by the French as part of their Indochina colony, only to be taken by another imperial power in the form of Japan. The Allies had announced that their motives for going to war were to ensure the survival of democracy and self-determination. Therefore it was understood that following the conclusion of the conflict, Vietnam would be granted independence. This, however did not happen, and nationalists in Vietnam from across the political spectrum banded together first to overthrow the French colonial regime in 1954, then an American occupying force. National liberation as an ideology is an extremely powerful force, one that has the ability to withstand any number of military powers as seen by the historical conflicts in Ireland, Algeria, Spain, Greece, the Ottoman Empire, and Afghanistan, among many others.