I mean the divergence apparently happened after WWII and the Cold War never ended. I think it's not ridiculous to assume that Russia and China turned out in economic and political structure to be communistic.
In Fallout 1, there isn't any hint of the great war being a cold war that never ended. All it's say is the great war was about natural ressources being almost exausted. Of course, it's not ridiculous to think that russia and china might have stayed communists, in label if nothing else. We just don't need to know that, or anything too accurate about any pre-war politics/old propagandas subjects, if you ask me. And more importantly, the characters of Fallout universe shoudn't know or care either.
I mean, yeah it did, but it was the way it was handled is the difference to me. Fallout might have always had the theme of letting go of the past but New Vegas really hammered down on that theme specifically. No questions about it.
It's true, Vegas has a big 'let go, and begin again' theme. I still have a problem with NCR claiming to be the new america, or Ulysses and his old america flag that the courrier might possibly use someday.
In Fallout 2, the NCR didn't need to resort to 'america, yeah baby' propaganda in order to be what they are, nor to know too much about the pre-war's life.
I guess the heart of the problem is that Fallout 1 never give me the feeling of playing a game set in america, it is simply the wasteland, post apocalypse, whatever. Geography aside, the plot could have been in europe, asia or wherever and it woudn't have mattered.
The same is almost true for Fallout 2, except for the damn enclave that I keep blaming for beginning this all 'america, baby, yeah fuck we are the US', that bethesda love.