New Vegas

Las Vegas was been mentioning by Tycho's father, who was also a Desert Ranger in Nevada. According to his memory, a fat mobster, similar to Gizmo from Junktown, used to live in Vegas. In the Fallout PNP game Las Vegas is home to the Church of the Mushroom Cloud, a cult which partakes in radiation baths and bleeding rituals, it has been suggested that the Children of the Atom may be a more peaceful offshoot or denomination of this church.

Behind the scenes
This is a reference to Fat Freddy, a character from Wasteland.