The domino theory really isn’t about military but poltical will, though at the time few people realized this.
The US got out of Vietnam when the North and South signed an agreement. The North broke the agreement and invaded. Congress, just getting over Watergate was in no mood for anything, much less to put money or troops back in.
The South’s government thought the USA would come back, at least with money or supplies, which didn’t happen. Even the North Vietnamese were take aback at how quickly they made gains
There was no political will. This example could also apply to after WWII. While Poland, Romania, and Bulgaria were certain go Communist, the Hungarians and especially Czechoslavakia had a real chance to be democracies. But the allies showed no political will to help Hungary which went Communist and after Czechoslavakia went Communist, there was an uproar, 'cause that shouldn’t have been.
Then later came wars in Greece, and Truman stepped in with the Truman Doctrine and Greece didn’t go Communist, although that didn’t translate to democracy.
This showed where there was will Communism could be halted and that is what started this domino theory.
The issues got clouded 'cause a lot of these places, it wasn’t so much about Communism it was about decolonization. Ho Chi Min expected the American to be on his side. Indeed Stalin was greatly able to play off Churchill against FDR and Truman because America was so anti-colony. As Churchill remarked without our colonies Great Britian goes from a world class power to a second or third rate power.
Would Thailand went Communist? I think there is a chance that if Ho Chi Min got into control of all Vietnam earlier Communism would’ve taken root in Vietnam, Cambodia and Loas, and we may have seen the Vietnam War taking place in Thailand.
America would’ve had interest in not letting the Thais fall, and the British had a beaten a war in Malaysia and Indonesia with Western backing got rid of Sukarno who was definately a pro communist.
So the domino theory showed, if you had the political will you could takes steps to stop it.
I think without Vietnam it was more likely that we would’ve seen other proxy wars elsewhere. Perhaps the Soviets would’ve supported Katanga (unlikely but…) or Biafra in their bids to be independent African nations with proxy wars lasting much longer.