Basing anything in FO on 'real life history' is vastly futile. Since USSR doesn't feature in the timeline, why assume they had any nukes at all. It's entirely possible that the FO Soviets were in the position of the Chinese in RL: having neither the warheads nor the launch systems to pose a realistic threat of MAD. IRL, PRC are a third ranked nuclear power, behind the minor NATO nuclear powers (UK, France) in capability.
Possile, hadn't thought of that before, but i was building on the hypothesis suggested by another forummer about the USSR Nuking all of China along with the USA, leveling it completely. Still, it's unlikely the USSR wouldn't have figured out how to build Nuclear weapons eventually You musn't forget that the earliest time in history the Fallout Universe could have devirged from ours, is directly after World War II, yet a russian spy at the Manhatten project dellivered the Soviets all the information they needed to build their own Nuclear Weapon. And even if the russian espoinage of the U.S. Nuclear weapon's development didn't occur in the Fallout Universe, it's still a fact that the USSR still was one of the largest powers in the world during Wolrd War II, retaining all the conditions (military might, recourses, popoulation etc.) for this power after the War, while China was a country of similair size which also managed to build a gigantic amount of Nuclear Weapons on their own as well. So it's very well possible the USSR accomplisched identical results.
Given the worldwide total destruction outlined in the background blurb, the only reason D.C remains as intact as it is has to be 'so we can play there'.
It seems most of us agree on that.