Aside from the theme, there's not really much in the way of similarities to compare and contrast here.
Vegas Showdown has each player building their own casino, and trying to manage cash flow and growth. It's a bidding, tile laying game that is a bit like Alhambra done more interestingly.
Las Vegas is a pure dice roller. There seem to be some interesting choices, but it still is "just" a dice game.
I've heard Showdown compared to Princes of Florence - and I can see that. I am a big VS fan.