The official overview of Fallout: New Vegas from Obsidian/Bethesda:
Experience all the sights and sounds of fabulous New Vegas, brought to you by Vault-Tec, America's First Choice in Post Nuclear Simulation. Explore the treacherous wastes of the Great Southwest from the safety and comfort of your very own vault: Meet new people, confront terrifying creatures, and arm yourself with the latest high-tech weaponry as you make a name for yourself on a thrilling new journey across the Mojave wasteland. A word of warning, however - while Vault-Tec engineers have prepared for every contingency,* in Vegas, fortunes can change in an instant. Enjoy your stay.
* Should not be construed as a legally-binding claim.