Where are you getting this? "Las Vegas" simply means "The Pastures".
Well, here was my thought: the city had historically been a place that the surrounding empires (Deseret, New Mexico, California and presumably Bajo Colorado) exiled their people based off the myths about the "City of Sin". The Houses took control of the region, claiming the remarkably intact ruins of Vegas as their own. They started a minor hydraulic empire based off of Lake Mead, conquering the surrounding tribes and minor towns. They remained a center for exiles, and then a center for trade. It became a legendary city for the Dineh, and was one of the holiest "Vortices" (more on that in a later post) on the way to the second holiest Vortex in New Agedom, Fifty-One. Its greatest industry, in addition to Dineh pilgrims, was mining of the ancient steel that underlined the city.
It goes through many ups and downs, conquering stretches of the Colorado, maybe even setting up a Dineh dynasty at some point. But, eventually, the happy-happy good times come to an end with the collapse of the Hoover Dam. This throws them into turmoil, undermining goes into overdrive to purchase water to keep armies alive. All culminating in a final climactic battle between the casinos. The great thundering of feet, the great siege engines, and the blood leaching into the ground make it so something's gotta give. And it does, and the city collapses.
As to what it is in the modern-day: the territory itself is a vassal of the Dineh, consisting of a few minor farming villages and trading outposts. Some new town likely serves as the major stopover for pilgrims headed to Fifty-One, though it certainly doesn't attain the fame nor prosperity of Old Vegas.
The city itself is a crater, forbidden to all save the exiles, who are forced into it by the superstitious tribals at knife-point.
