As it says on the tin. People are still flocking in to blow their wages at the casinos, catch Wayne Newton at the Sands, or just eye the kitschy architecture, but you can also get easy access to not-quite-legal psychedelics on every street corner, cruise the proudly free-wheeling gay bar scene, or hook up with one of the New Age communes that are permanent fixtures on the city's outskirts. And it all just kinda jives together effortlessly.
Oh, and it still has to be located where Las Vegas is now.
One thing I could think is to maybe increase the number of universities in the area, give the city a few more insitutitions that attract the type who made Berkeley so famous in the 1960s. Also, maybe if the mob takes a greater business interest in the kind of drugs that were popular with the counterculture. Though that might run the risk of alienating Middle America, probably the biggest customer for the casinos.