As an adolescent of the 1980s, I was scared crapless at the thought of nuclear war. Back then, all that was needed was a damn good excuse to hit that button...which, thankfully, didn't happen. Russia's still on the 'crazy' list with Putin pulling Medvedevedevedevedev's strings (never know when to stop writing that name)...but he's not Kim Jung-Il 'crazy' or Mahmoud Achma-didn't-ever-see-WarGames-izhad 'crazy'...so a full-scale war...not likely.
Limited? Well, that may be a frightening possibility. Iran or North Korea may decide a 2-megaton explosion might be a grand idea on Israel/South Korea. If THAT happens, we're gonna have to go all "Modern Warfare" on their asses. Remember: Iran and Iraq are NOT friends...haven't been in decades. And, with Iraq now vaguely under U.S. control, this would be an interesting way for U.S., Iraqi and insurrgency soldiers to unite against a common foe. As for the PRK, even CHINA doesn't want them to have nukes...so what do you think'll happen if they set OFF one...ANYwhere?! The 300,000 troops China sent to help North Korea during the 1950s war would be NOTHING compared to the march from the north they'd get if they nuked someplace.
No need for Vaults or PIP-boys in this case. Seems most likely that Mother Nature is gonna destroy mankind before we do it to ourselves.