I just came back from Las Vegas, and the tone of the town never ceases to amaze me. A city always has a vibe, and for the most part, it's almost always the same. There are exceptions, but Vegas is by far the most different in the US.
It's not about prostitution. That's actually illegal in the city limits. In fact, these days there is a remarkable shortage of people passing around the hooker cards you would have seen ten years ago.
You can still smoke in buildings, which is unusual, since I come from one of the first states to adopt anti-smoking laws inside of buildings, even in bars. But that's not what it's about.
It's not even about gambling, which you can do almost anywhere now. The state of Illinois has opened up to the idea in a major way, and you're hard pressed to find a bar that doesn't offer gambling in some way. Which isn't to take away from the fact that Vegas has a slot machine for any intellectual property--even THE WIZARD OF OZ and THE WALKING DEAD (and I'm sure there is a slot machine dedicated to THE FOUNTAINHEAD somewhere)--but it's just not that special anymore.
No, take a walk down the Strip, and you'll feel a different energy from any other city in America. Everyone's drunk and happy just to hang out in a city where you can get anything at any time. Hell, there's an M&M store, and it's not just a little corner shop. No, it's a giant store with several levels, and they pump the smell of chocolate out into the street to entice people to come on in. At any given hour, you can find some drunken woman hanging off of the statue of the yellow M&M guy from the commercials that they have outside. Nearly all of those women are not concerned with their mini-skirts riding up to give a pervert the chance to see what kind of panties they wore tonight, just so long as their friends get pictures they can post to Facebook later.
In any other town, everyone would be on guard, but in Vegas, almost everyone has a license plate issued from out of the state of Nevada (unless it's a rental car). This truly is a party town. Granted, there are still homeless people asking for money, but it's not enough to drag the vibe down. The out-of-towners won't allow it. They might even encourage it.