This is not a reliable source. This is a veterans group that is still fighting the war, but for the history of their conflict in Southeast Asia. The source alone should give you pause. Westmoreland was simply stating beliefs that can't be proven. This is a 'counterfactual' argument. It's impossible to prove or disprove, since we can't know what would happen, and generally leads to unproductive conceptualizations of the past. We can say what did happen, and explore the contingency of the moment (i.e. what possible alternatives were possible).

Those supporting the 'domino theory' will point to the fall of Cambodia and Laos to communist regimes after 1975. I want to briefly explain why that was so, and why it did not spread to other Southeast Asian countries, thus contradicting the domino theory.

Vietnam, Laos and Cambodia had a long history of interaction, tightened by French colonialism. These areas were grouped into one unit, Indochina by the end of the 19th century. It was messy though, Khmers lived in Vietnam (Cochinchina) and many Vietnamese lived in Laos and Cambodia working as colonial bureaucrats or merchants. France developed an 'Indochinese' identity while suppressing the three national identities. And, as communist networks emerged in Vietnam and Vietnamese traveled to Moscow for training, they also adopted an Indochinese identity. The Comintern saw Indochina as one unit, and was not a proponent of narrow 'nationalist' parties, and pushed the Vietnamese to form an Indochinese Communist Party (ICP). Ironically the colonialists, their native allies, and the communists all spoke of Indochina as one political unit! But the Vietnamese were the dominant element in both numbers and influence in all three.

During the First and Second Indochina Wars, the communist Vietnamese therefore saw Indochina as one great battlefield, even if they had decided to organize along national lines (something that displeased Moscow) when they declared independence in 1945. The French and their local allies did the same, building three separate Associated States in Vietnam, Laos, and Cambodia. The Democratic Republic of Vietnam, dominated by the ICP, went about forming militant front groups, the Khmer Issarak and Lao Issara, and corresponding communist parties. Both were subordinate to Vietnamese support and instruction. The DRV allied with strong local elites in these countries, including members of the royal family. The Geneva Accords in 1954 halted these close contacts for a time, but they were renewed during the Second Indochina War, when the DRV likewise went about organizing and supporting resistance in Laos and Cambodia (though they struggled to control the new Khmer Rouge organization) and built the 'Ho Chi Minh trail' through both of those countries.

The conclusion: the spread of communist regimes into Lao and Cambodia was the product of longstanding ties between Vietnamese, Lao, and Khmer states and their revolutionary elites. These conditions did not exist in other countries. So you could say the domino theory was proved true in Indochina, but not in the rest of Southeast Asia ... but that in itself would disprove the logic behind the domino theory.

Christopher Goscha, Going Indochinese: Contesting Concepts of Space and Place in French Indochina (NIAS, 2012).

Nope. If anything, the opposite is true. Prior to US involvement in Vietnam, Laos and Cambodia were run by pro-American monarchs, but the US war spread into those neighboring countries and destabilized their regimes, leading to the success of communist regimes (the Pathet Lao and the Khmer Rouge). Heck, Vietnam wouldn't have been communist if we would have not stabbed Ho Chi Minh in the back and financed the brutal restoration of French colonialism instead of letting him form the democracy he wanted to form right after beating the Japanese in WWII. The source for that was a speech by General Westmoreland in the 1980s. I'd hardly consider a speech by the top vietnam general, cited on a website that is filled with ill-cited, factually questionable assertions a terribly accurate source.

For what its worth, The Philippines, Indonesia, Malaysia, and Singapore all had authoritarian, pro-western governments were at no risk of turning communist and never had serious communist insurgencies.

What's interesting to note is that Robert McNamara, after resigning from the Nixon Administration, noted that the Domino Theory had been wrong and that the indispensable country in Southeast Asia was Indonesia, which had been secured under Suharto.