An Intercontinuity Crossover implemented by way of two (or sometimes more) characters from different sources getting married while drunk in Las Vegas. Almost always starts off with the involved parties waking up the next morning and discovering, in order, 1) their hangovers, 2) each other, 3) their nudity, and 4) their wedding rings and/or marriage license. Photos and/or video of Elvis officiating at their nuptials are optional, but always appreciated. Hilarity Ensues as they attempt to reconstruct what happened the previous night (in addition to the obvious) and reveal/obscure their respective histories and secrets to/from each other.
Most often results in a decision to stay married, at least temporarily, until some issue or question is resolved. The crossover elements are further brought into play when explanations must be made to their friends and families.
In most cases the story assumes the two continuities have always existed together in the same world, and have just not intersected before the happy occasion.
This is a relatively new subtype of crossover, apparently born within the last few years. It seems to have first appeared on the Buffy the Vampire Slayer fanfic site Twisting The Hellmouth (which you may have noticed due to the below), but stories of this format have been spotted in other venues such as fanfiction.net, suggesting it is an actively-growing meme.
- Bad Luck, a Buffy/Teen Titans cross in which Xander and Jinx wake up to find themselves married. Unusual for a Vegas fic in that there is a dimensional transition involved as well.
- What Happens in Vegas Doesn't Stay in Vegas is a one-shot where Sasori moves to Vegas when the city of New York sues him for turning the kids into puppets. After a drunken night, Sasori ends up married to... Haine Otomiya from The Gentlemen's Alliance Cross of all people. Their marriage doesn't end well, to say the least.
- A Malfoy's Mistake. Draco Malfoy and Willow Rosenberg wake up to a new world together.
- Vegas Wedding. Faith Lehane and Daniel Jackson.
- All My Outers by PsychoSama is similar: Ranma Saotome gets married to Hotaru Tomoe, Haruka Tenoh, Michiru Kaioh, and Setsuna Meioh in a Muslim country (as Vegas is not somewhere you can marry four girls at once). Oh, by the way, a love potion was involved, but then again, Ranma's involved, so this isn't too surprising...
- Happened in a real Intercontinuity Crossover in 2004 when Wolverine married Sara Pezzini, bearer of the Witchblade, in Las Vegas. Of course, many such crossovers don't affect the continuity of the main books (and in fact the two had already crossed over in an in-continuity story, which this story ignores).
- Accidental by Jinni (at Twisting the Hellmouth): One of the first, and best, examples of the trope: Willow Rosenberg of Buffy the Vampire Slayer wakes up married to Sam Seaborne of The West Wing.
- What Happens In Vegas  by Silently Watches: Willow Potter (A genderbent version of Harry Potter, obviously) gets dragged to Vegas by Tonks, gets drunk, and ends up married to Raven of the Teen Titans.