It really is what it says on the tin. It's a risk for the U.S. to be economically dependent on a state whose government does not share American values, and that is threatening its neighbors with invasion.
Mexico is the USA's second largest trading partner worldwide. Exports of goods and services to Mexico support an estimated 1.1 million jobs (Department of Commerce). Mexico is also the second largest source of foreign crude oil to the USA, and the top export destination for US petroleum & natural gas. The US-Mexico-Canada agreement (USMCA) entered force in 2020, replacing NAFTA. The countries have a variety of complex ties. There are other treaties and areas of cooperation too, such as the International Boundary and Water Commission (IBWC). A security cooperation agreement called the Bicentennial Framework for Security, Public Health, and Safe Communities went into effect in 2021.
As for Vietnam, the US and Vietnam established a bilateral trade agreement in 2001. The US provides maritime security assistance to the country through the Maritime Security Initiative, Cooperative Threat Reduction Program, and through foreign military financing. US transferred Coast Guard cutters to Vietnam in 2017 and 2020 to bolster its law enforcement capabilities.
Those sound like alliances to me, but I'm interested to hear what other definition you're using.
Therefore psychologically and in terms of US foreign policy, those countries are more allied to the US then China.
It's not black and white. It lives on a gradient.
The entire point of formal alliances is that they are sworn, binding guarantees that can be relied on, not wishy-washy resolutions.
Think about it. It's the other way around. Reality is not defined by the word alliance, clearly there exists a gradient of countries who are more friendly than others.
Stick with the intention of the word by the person using it not the absolutist pedantic definition.
If you want to redefine ally to mean something closer to "partner", then Vietnam is also a "China ally" because it has a good partnership with China. Mexico could also be called a China ally by this loose definition.
But if you follow Vietnamese IR, you would know they take geopolitical neutrality very seriously, due to drama of 20th century. Both US and China separately invaded Vietnam because they thought it a Soviet proxy. No chance of Vietnam becoming an actual ally of either in the short term.
Similar for Mexico. US-Mexico relationship is extremely complicated right now. Right-wing US calling for invasion of Mexico on "drug war" grounds. Mexico purchasing and nationalizing Texas oil company, etc.
Vietnam: 84% have a favorable opinion of the U.S., 11% have a favorable opinion of China https://www.pewresearch.org/race-ethnicity/2023/07/19/vietna...
Mexico: 60% favorable of U.S., 30% unfavorable. For China, 57% favorable, 33% unfavorable https://www.pewresearch.org/global/2023/06/27/overall-opinio... https://www.pewresearch.org/global/2023/07/27/views-of-china...
The government of Vietnam has emphasized that it will not "choose" between US or China. ASEAN nations in general feel this way. They oppose war itself, and can't be tricked into taking sides in one instigated by Washington (or Beijing).
> Vietnam: 84% have a favorable opinion of the U.S., 11% have a favorable opinion of China
Please actually read the articles you link. This is for Vietnamese American citizens, not citizens of Vietnam. It is utterly unsurprising that immigrants have a overwhelmingly favorable opinion of their new home country, and reveals next to nothing about the PRC-SRV relations .
The numbers for Vietnamese citizens are similar. Vietnam one of the most pro-American, anti-Chinese countries in the world. 76% favorable for the U.S., 78% unfavorable for China, though the dataset is older. https://www.pewresearch.org/global/2014/07/14/chapter-1-the-... https://www.pewresearch.org/global/2014/07/14/chapter-2-chin...
We've gotten a bit far from the original point - for the purposes of the U.S. supply chain, Vietnamese government neutrality and strong public support is plenty to negate the risk that the country suddenly cuts off the supply chain. My original point is that de-coupling from China is a national security decision, not protectionism, and production is moving to Vietnam because it's a friendlier country than
Informally, I do think Vietnam can be considered a U.S. ally. China is signaling a desire to expand, and most of its neighbors consider it the biggest threat to their security. There's a reason that Vietnam, Japan, South Korea, and Taiwan are all among the countries that have the highest opinion of the U.S. It's not a trick - they are acting in their own security interests.
Vietnamese neutrality makes perfect sense, and I don't think they'd get involved in a war that didn't involve them. But the country is certainly friendly enough to rely on for the U.S. supply chain.