Adding on, does the above trend show a growth in sympathy for pyschological trauma? Or rather does it parallel how those Americans wanted to view/ potray the war, namely as tragic and shameful?
There are several factors to this. One, is the nature of films themselves. I won't try to delve into that history, as its not quite my area, except to say that portrayals of the psychological effect of war on soldiers was routinely not shown or heavily toned down, as was the gruesome violence of the war as well. In certain cases, though, we can see great attempts made at showing the "other" side to war. A great example is The Twilight Zone. Rod Serling served in the Pacific theater and was subsequently traumatized. He used this to give more nuanced portrayals of war. For a beautiful example, see "Death's Head-Revisited."
Secondly, though, there are several reasons Vietnam films went further into the psychological aspect. PTSD was only classified in the DSM III in 1980 (James Wright, Enduring Vietnam, St. Martin's Press, 2017). Certainly, veterans experienced psychological trauma in every war, but the major change taking place in Vietnam was that soldiers and Marines were simply surviving more than ever. Medevac typically was able to get wounded to a hospital in under an hour, and a wounded man who lived through the first 24 hours, had a 99% survival rate. So, guys in Vietnam have now lived through trauma that would have been more than likely fatal in previous wars. Vietnam also had the significant difference in that there was no completely "safe" area in country. The only ground you controlled what what you stood on, and even that might not be true. That led to a constant state of fear and apprehension in many, from front line infantry to rear echelon desk guys. Attacks were rarer in the rear, but present enough that one's safety could not be guaranteed.
Added to all of this is that by the early 1980s, there had largely been a gap in war movies depicting Vietnam. With the Wall being placed in DC, America began more and more to open up to such works. I would scarcely say anyone in 1975 thought a movie depicting a platoon in Vietnam that depicted atrocities and the gruesome torment of the war would ever win a best picture Oscar. There's more I could add, but its after midnight here.
Sources: James Wright, Enduring Vietnam See also Peter Kindsvatter American Soldiers:Ground Combat in the World Wars, Korea, and Vietnam University Press of Kansas, 2003
How would the Vietnam war be measured to other wars to be classed as one of most destructive psychologically? Would it be by percentage/sum of soldiers suffering from PTSD? Is effects on culture during and after the war counted?
I've also seen a similar focus of the trauma of soliders after WW1, as opposed to that of WW2 or the civil war.