As we all know, IOTL South Vietnam fell to communist insurgents in 1968. It was one of the few black marks during LBJ's 9 year presidency which is widely considered to be one of the best in American history. What if we had intervened in Vietnam just like we did in Korea? Would we have saved Southeast Asia from communism? Would it have overshadowed the Great Society?
Despite the patriotic fervor of my colleague here, I would say I am grateful we did not get involved. Namely because of the elephant in the room known as China. China would've likely gotten involved in Vietnam if we did and while South Vietnam fell to communist insurgents, it was still pretty unstable because of the terrain. The Chinese soldiers sent to reinforce the new regime died in startling amounts and South Vietnamese insurgents used guerrilla tactics to the fullest. When you look into it, it's a surprising amount of people that died. Things did sort themselves out over the years and China pulled out, but imagine those sort of conflicts in war, especially for people not used to it like the Americans.
Beyond that, it was good we didn't get involved. The USSR Afghan War shows pretty much the political quagmire that could've happened and how the large increase of rebellion against the USSR for its policies led to its collapse. Granted the half reactionary and half liberal government they have now is still pretty uneasy, but it seems like things are getting better, especially since that KGB thug Putin was not elected.
Had we invaded Vietnam, I imagine it would set off a large anti-war protest, given how the Soviet people reacted to the horrors of the afghan war and the atrocities committed by the soldiers. This would've tarnished American reputation and have a lot of good men die for what reason exactly? Even going beyond the Cold War affiliations, places like Southeast Asia and such weren't fond of us because they saw us as extensions of imperialism that f*cked them over in the past. Granted, China's behavior toward them is a twisted reflection of American-Mexican relations (which has improved vastly since marijuana was decriminalized as part of the Great Society thing among other reasons) and is reminiscent on how the US exploited the Caribbean and Central America. Heck, it's why we're having better relations with them now than in the past.
Heck, our outstanding global reputation was why we were able to involved in the Middle East in the first place, especially after the death of the Shah. While we shouldn't have done Operation Ajax in the first place, at the very least, LBJ did help the new regime and we have better relations with Iran now, which is helping keeping the fragmented Arabia in peace.
But lastly, Vietnam itself is doing pretty all right and we're doing good with them. The two Koreas remain an issue, but the dynasty in North Korea lost a good amount of credibility once South Korea revived its economy and the fact that US involvement was only because of what they did in the past.
The Great Society was successful as it was because of the funds we poured into it, funds which would have been wasted on a war in Vietnam that no side probably wanted in the first place. While I'm sure it the Great Society would still exist even if we went to war, it would not be as effective. No universal healthcare for the US, least by that point, I can tell you that.