Have Jennifer Aniston and Vince Vaughn decided to pack it in after more than a year of clandestine coziness? That's the word from Us Weekly, which just two months after pronouncing them engaged -- a claim Aniston labeled "insane" -- now says they're kaput.

"Split!" declares the latest cover. "Vince takes back the $500,000 engagement ring as the couple suddenly cools ..."

Vince, 36, and Jen, 37, purportedly put a fork in the romance around Sept. 9, just before the towering funnyman jetted off to London to film "Joe Claus," in which he stars as Santa's crankypants older brother (pure comedy gold, people). The mag says they "solidified the breakup" during a lengthy phone chat.

"It's not common knowledge, but we're not together anymore," Vaughn is quoted as oversharing with a source on Sept. 13. "We've split up."

An Aniston insider concurs: "They're 100 percent done."

(By the by, just last week, a pal insisted to Us, "Jen and Vince are still 100 percent together," but moving on ...)

Vaughn, says the mag, quickly put the kibosh on Aniston's planned trip to London for a shindig on Sept. 23, just one day before she was snapped taking a solitary stroll with her dogs along the beach near her Malibu home.

The romance, which began in the summer of 2005 on the Chicago set of "The Break-Up" (the hit flick arrives on DVD Oct. 17 -- somewhere, a marketing department is doing a happy dance), reportedly began to fizzle "almost immediately after" Vince slipped a knuckle-crushing 9.5-carat canary diamond ring on Jen's finger as they winged home on a private jet from a Mexican getaway in late June.

"Vince eventually saw how needy she is, and it scared him off," an unkind snitch says of the former Mrs. Pitt.

Other issues the magazine posits played into Vaughniston's supposed parting: Vince's mother didn't give them her stamp of approval ("It was a real letdown for Jen," confides an insider. "She is upset his mom got in the way"), and the low-key actor had grown weary over the attention they attracted together.

Aniston's rep, however, dismisses the Us story as "rubbish" (via E!).

This isn't the first rumbling of trouble to hit the surreptitious squeezes in recent days. Last week, a Vaughn insider told People "the tempo has slowed" on their relationship: "They used to be inseparable. Now they have busy lives in different parts of the world."

Back in August, as Aniston laughed off the engagement reports to People ("I mean, it couldn't be more of a 'No.'"), she described her connection with Vaughn as "just being. We're having a good time."

And while they've wisely kept their pieholes plugged about the details of their relationship, that hasn't stopped them from gushing about each other's finer qualities.

"She's the most sincere woman I know. She doesn't lie, she has a good heart," Vaughn said of Aniston this summer while promoting "The Break-Up." "She's very kind, she's very aware, she's considerate to other people. She's a very warm person."

Jen, meanwhile, told the May issue of Harper's Bazaar that Vince is "a very deep human being, incredibly smart, loves history, is protective and very loyal to his family and friends."

And in what may bode well for their future, she added, "If you're his friend, it's for life, something I respect in a person."