Okay, just verified that, but it stinks of Houses tendency to self aggrandize, there is nothing in Vegas worth nuking.
Hoover Dam, the airfields of Searchlight, McCarran Intl, Nellis AFB, Black Mountain (actually nuked - there's a giant radioactive crater right in the middle of Tabitha's supermutant settlement)...
Don't forget RobCo Industries, one of the States' largest robotics companies, and REPCONN Aerospace's primary headquarters and testing sites in the region. If someone leaked the true nature of the related Helios and Archimedes projects, then that might have added even more impetus as a viable target. Otherwise, I'd say Las Vegas likely qualified as a secondary target, something to lob a few nukes at to disrupt any reconstruction, but not something worth completely glassing. Given the size of historical nuclear arsenals, however, a few extra nukes here and there were probably perfectly fine. It's probably fortunate for House as well; his system held up well enough, but against a total strike, it probably would have been overwhelmed.
All not Vegas. Those are separate and distinct targets from Vegas. Vegas is a city, not a region. Also, even Russia and China know there is no point in trying to nuke Hoover, the damned thing would take a fifty megaton device to the face and smile at you.
Not with a groundburst, and you can bet that they tailor the attack to the target instead of adopting a one-size-fits-all approach. Drop a sufficiently powerful warhead in the reservoir close enough to the dam, and let water shock do for you what the barrel bombs did for the Ruhr dams. Besides, the goal isn't to destroy the dam, but rather just to render it unusuable by the enemy; irradiating it would accomplish the goal as effectively as structural damage would.