Whether or not the domino theory panned out is a matter of opinion, and there's plenty of information on it online. But one must remember that Cambodia and Laos were also involved, and SE Asia is hardly a bastion of democracy. As for whether or not Viet Nam would have evolved as it has without our intervention, one must also remember that totalitarianism is self-limiting.
As for "Trump is right in what he says", what he says is based on the kind of shallow thinking of the typical Trump supporter and can change -- and often does -- daily. Given his paranoia, his fondness for conspiracy theories, and his knee-jerk foreign policy, he is very much a loose cannon when it comes to that area of the world in particular, and given the continuing possibility of war, he is hardly an American problem alone.
SE Asia does not have to be democratic. As long as the people, there, get on with life the way they want. At that time communism was a very real threat and we are talking 50 years after the fact. Most of us were kids at the time. I was in my twenties. I was barely 20 when China became communist. and we were all very preoccupied with the red wave that was spreading from Moscow, across to Hong Kong and Korea. It, already, had a good chunk of Europe. So much for self-limitation! Whether they did some questionable things, or not, Thank God we had the US to go into Korea and Viet Nam. History books can be very unkind.
Different times. To-day, there are different problems to solve and, although Trump may be the wrong choice, we are all going to have to change---as, you, yourself have said. That is the danger of something new. In Spain, we have a couple of new parties, born with the same discontent that you have, over there.
People are not communists, but the parties that emerge have to be watched, with regard to who the leadership is. They have a vey democratic way of getting into power. Getting them out, again,is a different matter. Putin is an example.