Stop me if you've heard this one before.
Preparation & Setup
Loud & Dumb
Cash out your chips in grand fashion by committing a war crime and flooding the building with tear gas. Rush the vault and blow it open with enough explosives to have them feel it in Las Vincents.
Grab yourself some military quality tear gas off of a military convoy dumping it in the desert. While you're at it, get some gas masks so you don't spend the entire heist sobbing your eyes out. Even better, acquire some military grade rifles along with some explosives so you can actually get into the vault.
Smart & Subtle
Cash out your winnings in a way that will ensure you don't get jumped in the alley behind the casino. Blackmail a dealer to get you into the vault and deal with the nastier aspect of casino robbery after you leave the building.
Get yourself some uniforms for the casino along with a sympathetic employee made sympathetic by some light blackmail. Smuggle in some firearms & explosives in case shit goes sideways.