Las Vegas is a risky venture, IMO. The NBA will move there, and I think they have a shot at success. They are more of a maintstream product, and can probably make it work.
Las Vegas is one of the fastest growing cities in the U.S., however, that growth is in through the casinos, and the related tourism. You can't sell season tickets to tourists. With the casinos being by far the largest industry in town, by far, you have a lower income level. Casino jobs don't pay great. They rake in money, but most of that money flows back into the investors, outside of the city.
That leaves the casinos themselves as your biggest clients. There is an appeal there. It gives them something to offer their big rollers that's different than stuff that's currently on the strip. However, location becomes a big worry. Casinos don't like sending their big customers away from the gambling too far. They want them close by, so that they can hit the tables right after the entertainment is done. That means getting an arena right on the strip, which is prime real estate. That can get pricey.
I say, wait for the NBA to move in there. They'll build an arena, get a location, build a relationship with the casinos. If it's successful, the NHL moving in should be much easier. They can piggyback on their success, and whoever owns the team probably wouldn't mind having another major tenant to share the costs.