Las Vegas is Las Vegas because it has the perception that "anything goes". So far Dubai has no casinos, a no-skin dress code, no alcohol in public, and they're cracking down on kissing. I'd say Vegas is safe for now.
Vegas isn't in any danger of being replaced by Dubai of all places. Vegas is vegas in part because it is shielded by the laws of the USA. Unlike Vegas, Dubai is controlled by the laws of Sharia as interpreted by the ruler of the day.
Well, it doesn't have to be the Las Vegas of the world, just the Las Vegas of the Middle East. But yes, obviously religion is the factor that holds it back. If the emirs (correct title?) can get it into their heads to let money beat religion, though, they have a huge opportunity.
There's definitely a future for Dubai as the Las Vegas of the middle east, if that's where they choose to go.