Don't shoot me, but i found the different cities in fallout 2 quite nice and new reno a very interesting town, not just the way they did the quests. Basically, if it doesn't blatantly insult your intelligence, some inconsistency isn't always a bad thing.
For example, new reno was fine for me, san Fransisco on the other hand i didn't like at all. It's all about how you do things!
You can change scenery every 5 meters and don't care at all and you can keep the same setting in your entire game and still feel that it is lacking!
Anyway, that said, it should have gambling, it should have prostitutes and all, not because it is las vegas, but because it is a destroyed world and ethics in a destroyed world don't count for much. No, i don't want for example nudity for the sake of nudity, but it should have a dark atmosphere, that even "good" places aren't holy, with the sun shining and the birds singing!
About how vegas building style should be... I'll leave that to the experts

Joke aside though, I do want one thing. If it is many decades after the war, it should show some evolution from the citizens that live there. Reasonable evolution. Don't show repaired buildings with technology that came right through top-notch construction yards,but also don't make them live in buildings that feel that they would crumble in the next second. Materials should have been salvaged and used by now!