Nuclear Holocaust has holes and variables. Vegas is not a strategic location, and would have likely died quickly after the rest of America was wasted, therefore little to no tonnage would be wasted on it. A perfect example is the Pitt, witch was mostly unbombed, of course, unlike Vegas, witch is well distant from waterway, The Pitt's rivers turned it into a nuclear heatsink. On the other hand you have San Fran, witch is the literal economic gateway to the far east, and location of stratiegic nuclear defence, that in the Fallout Universe, is likely still very active by that time, making it the best place to dump a load of fission.
No one will bomb the arctic poles, so if you ever get paranoid, invest in the Antarctic landmass!
Another variable will now be dubbed the "Megaton Settlement" effect. That one bomb in Megaton could have wiped out a great chunk of the city of DC, but was a dud. Enough luck and you could have whole metropolis area survive, although the chances are slim.