Test Drive Unlimited 2 is has a
huge casino with Roulette, slot machines, poker, poker tournaments and much more. You can even buy clothes, drinks, and cars in the casion. It even has objectives and multiple ways to level up. In fact its the best part of the entire game.
The only problem is the game is quite buggy. But if you want a casino type game, TDU2 Casino is about as good as it gets for the 360. In fact I think its the only game with an actual casino.
You might download the demo of
Fable II Pub Games and give it a spin. Haw haw. They are not traditional Vegas games, but the Spinner game is pretty much like slots, and I actually like the other two games.