"Had we not fought in Vietnam, where would the momentum of communism have carried it?"
Exactly where it did carry it - to the grave. Communism wasnt stopped with guns or bombs, economics is what killed it and what was always going to kill it. A beast like that dies more quickly in peace time (when people expect to eat) than in wartime (when they can easily be taught to blame their empty stomachs on the enemy.)
"Would a newly communist Vietnam, without the economic and military ruin of a long war have felt emboldened both by success and by ideology to invade Thailand? Malaysia?"
Vietnam was a nationalist struggle against the French, they 'turned communist' to get communist bloc weaponry once they were certain no one else would help them. They havent been aggressive outside their borders in modern history, the country was a shambles, and the entire idea sounds like something you would have to know nothing at all about the situation to take seriously.
Unless you are one of those people that likes to play with the meaning of 'impossible.' No, it's not impossible. Not impossible that the French will nuke us in the morning either, but I think it's a fairly safe bet. And remember you dont get to weigh some imaginary costless intervention against the remote possibility of something bad happening. Real invasions, at their best, are still very very bad. Lots of death and destruction and misery and lots of monetary expense. Not something you want to run around doing on a whim just because it's 'possible' that something bad might one day happen if you dont.
"Would Saddam Hussein have used oil profits from both Kuwait and Iraq to build a larger military to subdue Saudi Arabia, Syria, Lebanon and with enough money even Iran? "
Look at his track record. How many invasions did he launch? Two. How many did he get a US 'green light' on before he moved? Two.
He was a greaseball and a thug and not a nice person at all, but he could be and had been deterred very effectively, just like all the others.