The old Hollywood formula for romantic comedy was: Take two very good-looking stars, throw them together, tear them apart, let them surmount their obstacles and end with a wedding.

That was, of course, P.A., "pre-Apatow." The comic brand name who sometimes writes, sometimes directs and often produces has raised the bar on raunch and lowered the bar on "good looking," at least as far as romantic-comedy leading men go, with "Knocked Up," "Superbad," "The 40-Year Old Virgin" and "Forgetting Sarah Marshall."

So "What Happens in Vegas ..." is a real throwback, with a gorgeous leading lady, dreamboat leading man and a comfy PG-13 rating. Nothing that raunchy, or that funny, happens in this indifferently written wade through the shoals of love, marriage and divorce, Vegas-style.

That's where the micro-managing commodities trader Joy (Cameron Diaz), who has just been dumped, meets the free-spirit slacker Jack (Ashton Kutcher), who has just been fired ... by his dad.

Yes, they "meet cute." A mixup in hotel rooms sets up a vigorous assault by Joy and her pal Tipper (Lake Bell, funny) on Jack and his pal Hater (Rob Corddry, funny, too) because they think the guys are perverts.

After that is cleared up and after much alcohol is consumed in a blur of "Viva Las Vegas," Jack and Joy have wedding rings shaped like dice.

Sober, she's wound a bit tight, one of those "make plans to make plans" types.

"She's awfully hostile for a girl named 'Joy.' "

And he's irresponsible, "not exactly serious boyfriend or marriage material."

If only they can get an annulment. Quickly. If only he hadn't hit a slots jackpot just as they were splitting up.

One obnoxious judge (Dennis Miller, no acting needed there) and one dullard marriage counselor later (Queen Latifah, phoning one in), Joy and Jack are sentenced to "six months hard marriage" to see how the $3 million they just won will be divvied up.

A scattering of laughs interrupt the crickets through the middle acts as each tries to bait, taunt or trick the other into some act of bad faith that will void the marriage and win them the jackpot. Yes, we have our second pee-in-the-sink scene of the spring ("Baby Mama's" finest moment). Come to think of it, the toilet, toilet seat and variations thereof comprise too many of the jokes here.

The insults are mild — "Retarded!" "Robot!" So are the laughs. And Kutcher and Diaz, at least in the early Vegas scenes, seem like the sort of good-looking good-timers we'd all like to party with.

But screenwriter Dana Fox ("Wedding Date" was hers) and director Tom Vaughan ("Starter for Ten") can't find much fun or romance in this. "What Happens in Vegas ..." offers none of the surprises that screen comedy so desperately needs.

Kutcher's goofy charm wears thin. Diaz, who can lay it on comically and romantically with the best of them, doesn't get to play the perky, goofy, drunken Joy enough to give this life.

And Diaz almost certainly chased down the cinematographer who lit her casino scenes and gouged out his or her eyes with a light meter. She looks a fright there. And if we didn't read about her taking her revenge after the shoot, well, you know what they say about what happens in Vegas. ...