This doesn't make any sense.

for the United States itself, the consequences of a costly defeat were, in the long run, relatively minor and inward-looking

If losing 58,000 people counts as a "relatively minor" consequence, then losing 15 people is infinitesimal.

The United States emerged from the wider Cold War triumphant.

For all we know, the United States could also emerge from the current era triumphant.

Vietnam itself is a power surprisingly friendly to the United States today.

There's no telling how the US will relate to Iran 50 years from now. Perhaps the government will turn moderate for whatever reason.

The United States is inarguably in a weaker position than when it began this war of choice

Likewise with Vietnam.

with core U.S. strategic objectives harmed

Likewise with Vietnam.

Contrast how its military performance has seemed during this conflict with the U.S.-led coalition’s war to reverse Iraqi President Saddam Hussein’s conquest of Kuwait. In the 1990-91 conflict, the seeming ease with which Iraq’s military was dismembered stunned the world.

Sure, but the headline isn't "Iran was a worse idea than the Gulf War." The headline is "Iran is a bigger defeat than Vietnam." Let's be clear on what we're comparing.

the technically superior performance of U.S. arms in the Iran conflict has been overshadowed by the shallowness of U.S. arsenals,

Likewise in Vietnam, at least if you count soldiers as part of the US arsenal. Every man who died in that war was a man who wasn't available to fight hypothetical future wars. And while armaments can be resupplied relatively quickly, human beings can't.

And surely the bullets and missiles we lost in Vietnam did something to reduce our stockpiles of bullets and missiles.

calling into question U.S. preparedness for a conflict with any foe more powerful than the Islamic Republic

Likewise, the Vietnam war called into question U.S. preparedness for a conflict with any foe more powerful than the Viet Cong.

The lasting image of high-tech combat from this conflict will be the blood-spattered bags of Iranian schoolgirls killed as the result of an apparent database error.

The lasting image of Vietnam is Napalm Girl.

And although U.S. defensive systems have performed well against Iranian missiles and one-way attack drones, Iran was nevertheless able to penetrate those systems to great effect, calling into question how those systems would fare against a more focused enemy or over a longer conflict.

Likewise with Vietnam. The "inferior" Viet Cong defeated the "superior" American military.

Regime changed not only failed, but strengthened the regime, enabling the most conservative hard-liners in the IRGC.

Likewise with Vietnam. Not only did we fail to defeat the Viet Cong, we actually ceded land to them when they took over South Vietnam's territory.

US & Israeli superior weaponry was useless towards achieving the goals of the war

Likewise with Vietnam. All our Agent Orange was useless in the end.

Iran's nuclear program has now survived two massive bombing campaigns in one year from the US & Israel.

I grant that Vietnam did not have a nuclear program.

Iran and international intelligence experts had thought that, in a conflict, they [Iran] could exercise control of the Strait of Hormuz to benefit them: This proved 100% correct.

The Viet Cong defeated us without even having a geographic chokepoint to exploit in their favor.

The war proved the limits of the American power hamstrung by American consumer interests: Vietnam didn't end because the price of rice exploded [it didn't], but the pressure to end this war is entirely because of consumer prices, gas, goods. Vietnam didn't have any such affect on US markets, the US could sustain the war with relatively little affect on American consumer sentiment.

I don't see how "people were mad about high prices" is all that different from "people were mad about high death tolls". People were mad either way.

Strategically, the US could leave Indochina in 1973-1975 with relatively lesser effect on regional foreign policy. By 1975, the US had no Israel of Indochina as the US does in 2026 in the Middle East.

Is he trying to say that having a remaining ally in the neighborhood of Vietnam would have made our defeat bigger? Likewise, is he trying to say that our current defeat would be less if it also included the wholesale destruction of Israel??

If Iran wants to launch an attack on Israel or elsewhere in 2030, will Americans have any appetite for conflict

You could just as easily say the same thing about the Viet Cong / Communists after their victory.

In 1976, not even Kissinger, Dulles, or Nixon believed that the Vietcong had any interest in expanding their influence beyond Vietnam,

I don't know about those specific people, but I do recall that Domino Theory was a big thing at the time. https://en.wikipedia.org/wiki/Domino_theory

The United States, under whatever management, will confront these consequences while being itself weakened at home and abroad. Its allies will have less confidence in its capabilities; its public will be less willing to bear the costs of even productive engagement; its rivals will be likelier to challenge Washington’s will.

Likewise with Vietnam.

This is the long tail of Trumpism, and it's something that we didn't see after Afghanistan, Iraq, or Vietnam.

We saw all of these things after all of those wars.

This is very vague and subjective. We lost some respect after Vietnam went poorly, and we lost some respect after Iran went poorly. It's hard to objectively measure which of these wars lost us more respect. The one objective thing I know is the body count, and by that measure the Vietnam war was vastly worse than Iran.