Kinda contradictory to actual border locations as Vegas is relatively close to the California border.
Vegas is not all that far from Area 51. http://www.lasvegastourism.com/area_51.htm
Also there is a link between the 2, there is a terminal just for workers of area 51 as well as a jumbo jet. So it is by no means contradictory at all, as Zion is further away from Vegas than A51. http://usparks.about.com/od/travelitineraries/tp/Driving-Distance-And-Drive-Times-From-Las-Vegas-To-Selected-National-Parks.htm
Also Area 51 is not in any dlc, I asked and was answered during one of the q&a things for New Vegas. It svcks and is a major missed opportunity for them but whatever, they could be saving it.