Las Vegas, NM. Online Offers. Las Vegas is a city in San Miguel County, New Mexico, United States. Once two separate municipalities (one a city and the other a town) both named Las Vegas, west Las Vegas (Old Town) and east Las Vegas (New Town), divided by the Gallinas River, retain distinct characters and separate, rival school districts. The population was 14,565 at the 2000 census. It is the county seat of San Miguel County. Las Vegas was established in 1835 after a group of settlers received a land grant from the Mexican government. The town was laid out in the traditional Spanish Colonial style, with a central plaza surrounded by buildings which could serve as fortifications in case of attack.