Las Vegas is probably way better at water management than say the Bay Area, which most people don't criticize for this, even though we ship water from a billion miles away (https://en.wikipedia.org/wiki/Hetch_Hetchy).
1) Hetch Hetchy brings water to San Francisco from 167 miles away.
2) The Colorado River supplies 90% of Las Vegas's water and brings it from up to a 1,000 miles away.
3) Per person water use in SF for residential customers is 45.7 gallons.
4) Per person water use in Las Vegas for residential customers is 74 gallons.
So. No, Las Vegas isn't even close to being better at water use than the Bay Area.
There were artesian springs which served as a watering stop for wagon trains, and later the railroad.
But most of the valley is dry to the point of being parched.