As the result of Clark Griswold's diligent hard work, he is rewarded with a bonus from Frank Shirley, inspiring him to organize an extravagant celebration for his family: a grand vacation to Las Vegas to rekindle their love and commitment. While the idea fills Ellen and their teenage daughter Audrey with initial excitement, it soon dims when they realize Clark's choice of destination. In contrast, their son Rusty finds himself buzzing with enthusiasm, eager to experience the vibrant energy of the city.
Once they reach Las Vegas, the Griswolds enjoy a dazzling performance by the famous duo Siegfried & Roy, followed by a poignant reunion with cousin Eddie, Cousin Eddie, who is married to Ellen's cousin Catherine. Living in a unique setting north of Las Vegas—once a hydrogen-bomb test site—Eddie and his family embody a lifestyle adapted to their eccentric environment.
On a guided tour of the iconic Hoover Dam, led by Arty, Clark inadvertently causes a minor leak, which results in him getting separated from the group. To reunite with them, he faces the challenge of scaling the dam's scaffolding, relying on his wit and determination to navigate this predicament.
The very next evening, the family is astonished to find tickets to a Wayne Newton concert delivered to their hotel, accompanied by a dress specifically for Ellen. This unexpected gift leads them to the concert where Wayne, in an amusing twist, reveals that the dress was indeed meant for her. As he brings her on stage and visits their table, it becomes evident that the iconic performer has taken a liking to her.
With the family agreeing to a day apart, each member explores their personal interests. Clark becomes engrossed in the thrilling world of gambling, while Rusty finds a winning streak, thanks to a fake ID resembling Frank Sinatra. Meanwhile, Audrey bonds with Eddie's adventurous daughter Vicki and her friends, immersing herself in Las Vegas's spirited nightlife. As for Ellen, her newfound relationship with Wayne Newton hints at something deeper than just a concert performance.
However, as Clark's gambling spree escalates, his family—Ellen, Ellen Griswold, and the kids—grow increasingly disillusioned and ultimately decide to leave him behind in frustration. During this turmoil, Rusty stumbles upon a fortune, winning four cars at the slot machines, while Audrey teams up with Vicki to take on a job as a go-go dancer at a local strip club. Meanwhile, Eddie reveals he has hidden cash in his front yard and sees a chance to repay the Griswolds for their generosity. The two men strive to win back their losses at the casino but find that Clark's gambling addiction once again sabotages their efforts, resulting in a loss of Eddie’s money.
As the reality of his situation dawns on him, Clark realizes that uniting with his family is more significant than regaining their financial losses. He orchestrates a reunion, gathering everyone together from across Las Vegas. To turn their luck around, they pool their remaining funds and make a final bet on a game of keno. While doing so, they encounter an elderly man who shows kindness and offers encouragement. Clark, extending an olive branch, invites him to join their makeshift family for the night.
In a serendipitous twist, this encounter leads to a remarkable turn of events. The old man wins the game, but tragically slips into unconsciousness during his moment of joy. Ellen quickly sends Rusty to find help, but by the time paramedics arrive, the man is declared dead—his name was Mr. Ellis. In his final breaths, he utters a mysterious message to Clark: "take the ticket." The casino staff retrieves the winning ticket, leaving the Griswolds in disbelief yet thankful for the change in their fortunes.
With their luck finally shifting positively, Clark and Ellen reaffirm their wedding vows in a heartwarming ceremony surrounded by Eddie's family. In a gesture of camaraderie, Clark generously gifts Eddie $5,000 in thanks for his friendship. As the Griswolds prepare to leave the dazzling lights of Las Vegas behind, they set off on their journey home in style—with Rusty's luck granting them four vehicles: a sleek red Dodge Viper, a maroon Ford Mustang, a black Hummer H1, and a white Ford Aspire.
