If you know what to look for, you can find indications that there were substantial numbers of vets from both world wars who came home with a lot of problems too. I think that a lot of them just burned out without much notice by society, especially the ww1 guys that came back to deal with a depression.
We also have to think about how many men we put through Vietnam. While the size of our military may not have compared to the number of men we had fighting ww2 at any one time, the war itself went on for ten years and like the French "moria" system in ww1, we spread the fun around to a lot more guys with shorter deployments.
Heck, I think a lot of that also had to do with the way our guys left Vietnam. Most left it about how they found it, an undecided warzone. The pullout followed by the fall of SVN denied our guys the opportunity to see a job finished properly. We left too many loose ends.