I’m not sure economic capacity is a useful metric in determining the validity or invalidity of the domino theory, particularly the local economies of the nations involved. North Vietnam’s economy was pretty much irrelevant in determining the outcome of the Vietnam War, as was the South Vietnamese economy. On the one side the war was fought with the US war economy, and on the other with China and the USSRs. The Soviets and Chinese were able to match every US escalation of the conflict by escalating their own supplies of war materials to North Vietnam. The final offensives of the war, the abortive Easter Offensive in 1972 and the final fall of South Vietnam in 1975 were both carried out by North Vietnamese forces utilizing conventional Soviet doctrine and equipment, meaning massed artillery barrages and large numbers of tanks.
US strategic bombing of the North led to Hanoi becoming the most heavily defended city against air attack in the world in terms of numbers of SAM sites and AAA batteries curtesy of equipment supplied to them from the USSR and China during the mid-late 1960s. Both the failure of the ARVN to stand up to the VC in open battle prior to direct US military involvement and the failure of Vietnamization of the war under Nixon demonstrated that military aid to the ARVN absent direct US involvement wasn’t going to keep South Vietnam from falling, no matter how much military hardware the US supplied to the South Vietnamese.
With your ‘worst case’ of the Thai and Burmese dominos falling, I wouldn’t count on it stopping there, and again the local economy of the nations involved would be pretty much irrelevant to the equation. Malaysia already had a communist insurgency that the British dealt with in the Malayan Emergency from 1948-60, but it reignited in 1968 and lasted until 1989. The Emergency (so called because the British didn’t want to call it a war) is often compared to Vietnam both in terms of similarities (the British used Agent Orange, internment camps, collective punishment and there were atrocities ala My Lai) and differences. One of the primary differences is that the Malayan communists had no source of outside support due to a lack of a land border to a friendly communist country, something that Thailand falling to the communists would have solved neatly.
Indonesia also had serious problems with communism, the Communist Party of Indonesia having 3,000,000 members in 1960, a problem that was ‘solved’ quite brutally with mass killing in 1965-66 which
According to the most widely published estimates at least 500,000 to 1.2 million people were killed,[3]: 3 [4][5][7] with some estimates going as high as two to three million.[15][16] The atrocities, sometimes described as a genocide[17][2][3] or politicide,[18][19] were instigated by the Indonesian Army under Suharto. Research and declassified documents demonstrate the Indonesian authorities received support from foreign countries such as the United States and the United Kingdom.
It isn’t hard to imagine a resurgence of communism in Indonesia following a fall of Thailand and Malaya (and likely Singapore as well) providing if not a land border but a short water crossing for supplies from neighboring now communist countries.
None of this is to say that I think the domino theory was valid in its own right (I don’t), but running with your argument of it working and continuing to tumble the Thai and Burmese dominos, there’s no reason to think it would have stopped there. Nor is there, in my estimation at least, much reason to think that the level of economic development in the local dominos would have made much difference since the local economies wasn’t fighting the wars, the superpowers of the Cold War’s economies were. The USSR might have had lines for all manner of basic commercial goods from food to clothing that led to its eventual fall decades after Vietnam, but it also had no problems producing mountains of war materials up until the day it collapsed.