By Glenn Whipp > Film Critic

A PG-13 sitcom rom-com that desperately wants to be a Judd Apatow movie when it grows up, Tom Vaughan's flat film mixes screwball comedy with self-help clichés to tired effect. Think of it as a "Green Card" aimed at the Planet Hollywood set, a movie that truly believes in its heart of hearts that gender relations could be dramatically improved if men would just remember to leave the toilet seat down.

"Vegas" opens with an aerial shot of Manhattan (an immediate signal of laziness), the home of control freak commodities trader Joy (Cameron Diaz) and layabout himbo Jack (Ashton Kutcher). Joy has just been dumped by her fiance. (She could never be good enough for him.)

Jack has just been fired by his dad. (He could never please him.)

These two damaged souls impulsively wind up in Vegas with their terminally unfunny best-friend sidekicks (Rob Corddry and Lake Bell) in tow. A few dozen cocktails later, they find themselves married. No problem. They'll get it annulled. Then Jack wins a $3 million slot payoff with Joy's quarter. Now we've got a predicament.

A judge (Dennis Miller) freezes the $3 million and sentences Jack and Joy to "six months' hard marriage." It sounds like that bad pilot episode George and Jerry wrote on "Seinfeld," the one with the judge ordering the guy to be Jerry's butler. Only "Seinfeld" spoofed the idea. Director Vaughan ("Starter for 10") and screenwriter Dana Fox ("The Wedding Date") crucially lack that self-awareness.

"Vegas" is divided into two halves. In the first, the unlikable Jack and Joy try to destroy the marriage. She lights votive candles, he pees into the sink. That kind of thing. There's a lot of forced wackiness and "snappy" repartee, like Jack asking Joy if she needs some help cleaning and then telling her, "Well, you should probably call somebody."

Then, the filmmakers abruptly decide that Jack and Joy should display likable human qualities. The movie actually improves here - Diaz and Kutcher can hold the screen - but you're well past the point of caring. The inevitability of their coupling is handled with robotic efficiency. The real happily-ever-

after arrives when the lights come up at the end.

Glenn Whipp, (818) 713-3672 glenn.whipp@dailynews.com