Are they or aren't they? Vince Vaughn has exactly zilch to reveal about his relationship with Jennifer Aniston, who co-stars in his oddly gloomy new romcom. But in every other respect you can't keep him quiet. Donald Clarke gets few words in edgeways while interviewing the frat pack funnyman

FOR THE last year or so, Vince Vaughn, a giant Chicagoan with crafty eyes, has been carrying out a fascinating experiment in obfuscation. In the summer of 2005, Jennifer Aniston, then recently cast adrift from Brad Pitt, was said to have been spotted spooning with Vaughn on the set of their new picture, The Break-Up. Since then, magazines have carried pictures of them shopping hand-in-hand. They couple have sat together at the final of the French Open tennis tournament. When they appeared together on the Oprah show - following a preposterous story that Ms Winfrey was to finance their wedding - the programme's production staff must have feared for the furniture. Mr Vaughn is nearly a whole foot taller than Tom Cruise, and if he jumps on a sofa it stays jumped upon.

As it happens, Vince, maintaining the enigmatic tone he has stubbornly held to since the story broke, barely acknowledged the existence of the relationship. Like those zealots in The Life of Brian who forbade believers from speaking the name of God, the actor will not allow the word "Vaughniston" to pass his lips.

One admires his discretion. But his advisors must have been concerned when he explained his strategy. An agent or a manager must surely have urged him to come clean with the media.

"I don't have that sort of relationship with my agents or manager," Vaughn says. "I don't have the sort of relationship where I allow them to handicap how I can run my personal life. I have always been very consistent. I saw myself as an actor and an entertainer and I kept my personal life to myself."

Fortunately, Vince Vaughn is one hell of a talker. Given the length of his answers and the speed of his delivery, any journalist seeking to nudge him towards indiscretion will need a great deal of patience (and a very long interview slot). Folded onto a generously stuffed sofa in London's Dorchester Hotel, sucking his way through a series of cigarettes, he listens to as few of my questions as he has to before launching himself into a succession of taut monologues.

The Break-Up, in which Vince and Jennifer play a couple ending their relationship badly, is quite an odd film. The publicity material suggests that the picture be viewed as a romantic comedy. But, featuring quite a lot of squabbling and more than a bit of cruel behaviour, Peyton Reed's strange entity is as gloomy a mainstream film as you will see this summer.

I know that Vince, who takes a producer credit, came up with the idea himself. Did aspects of break-ups in his own life inspire the scenario? Before I have reached the question mark, he is off.

"Yeah. I guess so," he says, taking a deep breath. "I would always get scripts for romantic comedies and they were always the exact same movie over and over. There was always some magical backdrop. Look, relationships are strange enough. You don't need to add some bizarre subplot. I thought it would be interesting to do something more heartfelt. I felt audiences might embrace something different and that would open the door for other film-makers with more different ideas."

The Break-Up sees the actor reprising a role he has intermittently returned to since his emergence in the now classic 1996 comedy Swingers. As in The Wedding Crashers, Old School and Made, Vaughn is the fast-talking wiseguy with an eye for the ladies.

Jon Favreau, who wrote and starred in Swingers, based the film, a tale of two actors bouncing around LA's bars and clubs, on his experiences hanging out with Vince. It is, therefore, not unreasonable to wonder what portion of the actor's patented lounge-lizard persona might have been fashioned from bits of the real Vince Vaughn.

"People do expect me to be like that. And there are sides of me that are like that. I have had the same friends for many years and I like to sit around and laugh with them and joke a lot. And, yeah, I have always liked girls quite a bit. I have always liked meeting girls. That stuff is true. But in movies, when you hit a strong part you can get associated with that. We all have different sides. No role can sum up a whole personality."

Swingers marked the end of the first section of Vaughn's three-act (at time of writing) career. Born in 1970, Vince was raised by his middle-class parents, a salesman and a stockbroker, in the suburbs of Chicago. He was a decent enough sportsman, but not good enough to consider turning professional. Then, one day, he accompanied a friend, a budding actor, into the city for an audition. For a lark, Vince had a go himself and, to his pal's chagrin, got the role. Hollywood beckoned.

"You are 19 years old and you suddenly say: I am an actor. You don't put limits on yourself. You don't have the wisdom to look at stuff that way. But I look back and am thankful I had the naivete to think I could make a living doing this."

Anybody who has seen Swingers will know what followed. Vaughn and his buddies sloped around Hollywood, scratching an existence playing bartenders in movies, dads in commercials and giant dogs at theme parks. Fun as that life sounds, there must have been moments when the boys contemplated failure. What if the big break never came?

"We were just thankful to get a television commercial, two lines on a TV show or one line on a movie," he says. "None of us really thought about starring in a movie. In fact, we never really believed we could make any money from acting. We were just doing something we loved to do. I think even if we hadn't made money, we would still be doing it."

Happily, Swingers made Vince Vaughn a name. In the career middle act that followed, he found himself playing supporting roles in successful films and lead roles in flops. He shared a jeep with Julianne Moore in The Lost World: Jurassic Park. He played Norman Bates in Gus Van Sant's puzzling remake of Psycho. It seemed as if he was destined to hang around as a reliable supporting player. Maybe, in his forties, he might get a sitcom on the telly. But it didn't look as if he would ever be a draw for moviegoers.

Then, in 2003, he wheeled out the lounge lizard again for the broad fraternity comedy Old School. The film was a hit and, in the years that followed, Vince has become the dean of that gang of comics - Luke Wilson, Ben Stiller, Will Ferrell, etc - known to enthusiasts of boozy humour as the Frat Pack. The team's antics in such films as Starsky & Hutch and Dodgeball have confirmed their status as box-office champs. But what about that middle act? Should he have done more comedy in the years immediately following Swingers?

"I am really thankful for that time, honestly. It was such a different time when I started. Now, I am only 36, so, I am not getting into a Little House on the Prairie thing: 'Oh, back then we had to walk 15 miles to school and back.' But I came out from Illinois at 18 and it was a different world then. At that time there was only one show on TV - Entertainment Tonight - that talked about celebrity. Now it's everywhere. I was grateful not to go through all that stuff immediately."

Which brings us back to his amusingly evasive locutions over the last year. As I write, the internet is awash with rumours that, shortly after Vince and I had our conversation, he got down on one knee before Jennifer. But both parties continue to dance around the issue in interviews.

"Well, I got to see a lot of other people - friends of mine - go through all this. So I learned a lot before I was in that position. I don't take any of it personally. I know that reporting this stuff is just a job. They are trying to sell a magazine or sell a paper and it really is not personal. They would do it to anybody.

"Actually, I have a laugh about it. I am curious to see what I am, apparently, doing next. It's like this soap opera starring myself."

The Break-Up opens on July 21st