As long as one of the PCs would be a traitor, go ahead and make 'em into a real one! Approach the player outside of the game and tell him a shady NPC approaches him with an offer he can't refuse...
On that note, you could just say one of the PCs has been under a high-powered charm or domination spell the whole game. Just reference some obscure saving throw they missed in one of the first sessions.
The VR one would be funny, I've been waiting to pull such a trick in a game I'm running. Even better you could have someone approach their PCs to tell them that they're beta-testing a new MMO game, something like Everquest 3, etc. Their PCs are actually beta testers who took the game too seriously and believed it was real...
(maybe their PCs are the players themselves who have taken the new video game too seriously, if they are big computer gamers and you really want to mess with 'em!)
Even better, tell them their characters are actually normal people who were delusional or high throughout the whole campaign. They've been wandering around the steam tunnels or the insane asylum from day one, thinking it was a fantasy world.
Deck of many things works. Just make sure they don't draw the card that gives 'em wishes or they can wish away the events caused by the deck.
If one of the players draws void, the campaign can end right there...