Las Vegas is the most populous city in Nevada, and is also Duke Nukem's home. The city is filled with buildings that have Duke Nukem's name, such as the Duke Dome, The Duke Burger, and The Lady killer Casino which has Duke Nukem's logo everywhere. Also, the city is filled with casinos, hotels, and strip clubs.
12 years after the defeat of the aliens, Duke Nukem became a multi-millionaire and Las Vegas became his home. The aliens, however, wanted revenge and a large portion of the city was infested or destroyed. The alien invasion ended when the Hoover Dam was destroyed.
In "My Digs" mode you can see Las Vegas rebuilt. My Digs takes place after Duke Nukem Forever, inferred from a news paper indicating that the aliens aren't leaving. You can see Las Vegas and The Duke Dome in particular which is now completed(during the campaign it was under construction).
The parts of Las Vegas seen in Duke Nukem Forever contain some more or less recognizable recreations of actual Las Vegas sites, most importantly a modified version of the Bellagio hotel with its famous dancing water fountain (the "Fellatio" logo seen in the Chapter Vegas in Ruin is most likely an allusion to the aforementioned hotel's name).