With both casinos and romantic comedies, we go in knowing better. We’re all aware that the odds are against us coming out winners, and that we’ll likely just become frustrated watching the same result happen again and again, with no validation for spending our hard-earned money. What Happens in Vegas takes its namesake a bit too seriously, never bothering to diverge from formula or make sure the customer ends up satisfied. The characters may leave Sin City as winners, but the audience exits the theatre in the red.

Ashton Kutcher and Cameron Diaz star as familiar stereotypes with no recognizable humans underneath. Jack is an affable slacker who works for his dad’s cabinet-making business, while Joy is a Type-A Wall Streeter living with her stuffy fiancé (Jason Sudeikis). When Jack gets fired and Joy gets dumped, they both decide, for varying illogical reasons, that Vegas is the only place to go. With their best friends in tow (loutish Rob Corddry for Jack, sassy Lake Bell for Joy), the two meet way-cute thanks to a hotel mishap, and spend the kind of alcohol-fueled night together that everyone in the movies has in Vegas, but no one in real life does.

When Jack and Joy wake up unhappily married, their immediate instinct is to divorce, but after Jack wins big on a slot machine Joy had been playing, the custody dispute over $3 million is more important. Going before a wildly implausible judge named, seriously, Judge Whopper (Dennis Miller), Joy and Jack are “sentenced to six months’ hard marriage,” forced to work it out thanks to some weird grudge the judge has against their generation. I know our legal system isn’t what it should be, but Lord help us if that’s the way justice is served.

As Jack and Joy try to undermine each other in their quests to escape the marriage and take the money for themselves, the series of pranks should be funny in a sparring, screwball-comedy kind of way. Instead, each plays off the most obvious stereotypes of gender differences. Women don’t like it when men leave the toilet seat up! Men can’t resist beautiful girls! This section is set-up for the inevitable ending in which Jack and Joy realize they really are meant for each other, but everything is so contrived that we never learn anything genuine about either person.

Playing an uptight character, Diaz never gets to cut loose in the party-girl way that has made her appealing in her most successful movies. Kutcher fares a little better in his usual wisecracking jester role, but he and Diaz are both weighed down by screenwriter Dana Fox’s screechingly awful dialogue. (“You bet on me,” Jack tells Joy near the end, “and you made me want to bet on myself.”) Corddry excels at playing creepy losers, but his dastardly lawyer goes too over the top to be fun. Bell valiantly overacts in an attempt to make her role more than just a stock sidekick, and even Sudeikis, who scored in a similar role on “30 Rock,” is nothing more than one of the many men-in-suits who populate lower Manhattan.

Director Tom Vaughan turned a trifle to gold with last year’s ’80s retread Starter for 10, but he’s at a loss here, unable to coax convincing performances or genuine emotion out of his leads or Fox’s screenplay. What Happens in Vegas is evidence that putting two attractive stars together onscreen is not enough to make a good, or even passable, 90 minutes of entertainment. The women at whom is this is aimed will find better romantic chemistry and more laughs in a repeat viewing of Iron Man.