The Yucca Mountain nuclear waste facility in Nevada was practically designed for Fallout (and fallout). They made that place to withstand nuclear war and survive for thousands of years, like a real Vault.
In RL, the project has been scrapped, but who wouldn't love to visit the miles of underground tunnels?
The Fallout Yucca Mtn story practically writes itself. I know it was built in the 90's, but the great war was in 2077, long after Yucca was built, so they can still fit it into Fallout canon.

Would make a good DLC I thinks.

I think Yucca is a bit far out to be in the Mojave wasteland. A DLC could feasibly visit it. Also, I think there's some other military installations in that area too.

You got me thinking now, I would like to see Yucca flats as a DLC maybe?

Area 51 isn't that far away.
Within the Nellis fly zone and, Nellis is in game.

http://www.aviationexplorer.com/AREA%2051%20PHOTOS%20AND%20MAPS/area_51_map_large.png