Even if it does stay in Vegas at first, it may show up on your doorstep if you are unwise enough to leave text messages about it on your cellphone.
On Point News reports that an Illinois woman sued her ex-boyfriend last week, alleging that he breached a promise to marry her and also breached a "fiduciary duty of implied fidelity" by hooking up with someone (identified as "Danielle") while he was in Vegas for his bachelor party. Plaintiff alleges that about 10 days later, "PLAINTIFF observed certain text messages from Danielle on DEFENDANT's cellular telephone," but that "DEFENDANT, seemingly believing that 'what happens in Vegas, stays in Vegas,' denied that anything happened ...." He later admitted otherwise, however. He also said the wedding was off, and that (plus the fact he was dating a lawyer) meant the lawsuit was on.
The Illinois Breach of Promise Act, passed in 1947, limits the damages available in such cases to "the actual damages sustained" as a result of the breach. (According to the statute, the act was passed because the threat of a breach-of-promise lawsuit had been used as "an instrument for blackmail by unscrupulous persons," which I am guessing means an Illinois legislator got caught cheating at some point.) That includes any money the plaintiff has already spent on the now-unnecessary wedding, but may or may not include the damages for emotional distress that the plaintiff is also seeking.
In this case it does not sound like the defendant actually brought up the "what happens in Vegas" defense, although he has not responded to the complaint yet. As we have seen before, this may have been a good ad campaign but is a bad legal defense. For example, it is not a defense to bigamy. Nor does it justify drunkenly demanding prostitutes be sent to you at work and then opening the door in the nude when the company's human resources director comes by to ask you to stop.
Granted, the casino that guy worked for was in Iowa, not Las Vegas, but it would probably not have mattered anyway.