New Vegas

Las Vegas was been mentioning by Tycho's father, who was Desert Ranger in Nevada also.

According to this memory, there lived a fat mobster, similar to Gizmo from Junktown. It is not certain, when Las Vegas Fat Mobster lived, because Tycho was educated by his father only and he probably didn't know his grandfather (Tycho's grandfather taught his son everything he knew).

This refference to Fat Freddy, the Wasteland game character, doesn't contradict Fallout canon. It is possible, that Tycho's family or even all Desert Rangers had some relations with postwar Las Vegas inhabitants.

Las Vegas localisation is beyond eastern edge of the Fallout worldmap (and beyond western edge of van Buren worldmap, 30 miles north-west of the Hoover Dam).

In real world Las Vegas lies on class I railroad from southern California to Salt Lake City.

According to the fan-made Jason Mical's Fallout PnP there are radioactive ruins only.