By 5:30 p.m., novelist William Styron had flown the coop, Peter Duchin's band was missing somewhere between here and the mainland and rock star Carly Simon, who is weaning her 2-year-old, had alerted her hostess that she would pop in for dinner only if the kid got exhausted enough.

"It's a little like Russian roulette," Jayne Ikard, co-chairman of the event, said coolly into the telephone at the "down-island" 19th-century Edgartown house she and her husband, millionaire Washington attorney Frank Ikard, had spent the past year restoring.

After six months in preparation, Friday was countdown night for the first islandwide, totally ecumenical $125-a-ticket benefit dance for the Martha's Vineyard Hospital and the Martha's Vineyard Community Services.

That may not seem like much of a challenge, but on the Vineyard, where "up" islanders and "down" islanders have little in common except the weather, getting them all to cross psychic boundaries takes some kind of nerve.

"Nobody ever tried to pull this island together before," said Jayne Ikard, a fearless ex-Bostonian who has been coming here for 20 years.

There was no panic about money. All expenses for the party had been underwritten. Ikard even hand-delivered last-minute invitations (original watercolor miniatures painted by her co-chairman, Raymond Ellis) to save on postage. The $20,000 in hand was pure gravy.

Still, the evening's guest list was very mercurial. Ikard already knew that "July people" like Walter Cronkite and Mike Wallace of CBS wouldn't be there; that former senator Edward Brooke and his wife were knee-deep in blueprints at St. Martin; that actress Ruth Gordon and her husband, director Garson Kanin, were question marks; that attorney Edward Bennett Williams was anybody's guess even though his wife, Agnes, had gone off to China; that columnist Art Buchwald was writing a speech; that nobody on the committee ("a bunch of extroverts, believe it or not") could work up the courage to invite playwright Lillian Hellman or actress Mia Farrow (she was trying on antique hats at Menemsha earlier in the day, while ex-husband Andre Previn waited with the kids out in the car,)

"It's kind of like deep politics,"Ikard continued, "and I wouldn't bet a nickle on anyone at this point. People come and go here like Air New England -- but with more regularity."

Somebody named it "Get Together '79," and for the 200 party-goers in the West Tisbury School gym, there was a distinct air of junior prom as well as class reunion to it. Fresh greenery mixed with tiny lights festooned the walls, and off in one corner stood the "bar," a solitary if bottomless bowl of untainted punch that was quickly remedied back at the tables by people who knew in advance that West Tisbury was dry.

On stage the Peter Duchin band, which didn't have to row over from Wood's Hole after all, was at it hot and heavy. Duchin was making do with an upright rather than the baby grand he'd requested ("I always ask for a baby grand," he said, wearily.).

On the dance floor and wearing shoes (the committee ruled out the high school gym when it learned that everybody would have had to dance in socks) was economist John Kenneth Galbraith, towering like an ambulatory lighthouse over Ikard.

In the arms of Washington attorney Ira Lowe, Rose Styron seemed to be doing nicely after all without her husband (off hunting overdue tax records at their Connecticut home, she said).

And Washingtonian couples like attorneys Nancy and Fred Dutton, Jennifer and Laughlin Phillips of the Phillips collection, socialites Nina and Michael Straight, Mary and Hollis Chenery of the World Bank and Aileen and Russell Train of the World Wildlife Fund bobbed on and off the dance floor with singles including Washington Post board chairman Katharine Graham, socialites Virginia Daley and Dorcas Hardin, journalists Carl Bernstein and Richard Cohen and Fairfax Hotel owner John Coleman.

Among the other notables present were MIT'S president Jerome Wiesner and wife Laya, author Vance Packard and wife Virginia and Time magazine's editor-in-chief Henry Grunwald and wife Beverly.

Carly Simon and Beverly Sills (with husband Peter Greenough) made it to the dinners given earlier by the Phillipses and Chenerys, but not to the dance. Even when those absentees were tallied up, some there Friday night like West Tisbury Selectman John Alley thought it "fabulous" if not miraculous that "up" and "down" islanders could peacefully coexist under one midisland roof.

"You have to be here 20 years before Alley will speak to you," said Vance Packard, who has been coming to the Vineyard for 28 years of five-month stretches.

"By rights, I shouldn't be here at all. I don't dance," said Henry Beetle Hough, first and foremost among island Pulitzer Prize winners since he won his in 1918.

"I suppose dancing is nice if you get used to it," continued Hough, 82, "but it's a helluva time for me to get used to it."

In his opinion, the party will have a drastic effect on future island social habits.

"What usually happens is that people come to the Vineyard to get away from everything, including parties like this," said State Sen. Chet Atkins of Concord. As chairman of the Massachusetts Democratic Committee, he knows a thing or two about fund-raisers on the island and said political ones, certainly, are usually a disaster.What surprised him about the dance, though, was that the pricetag was "so low," considering the potential some pocketbooks present offered. ("Not that low," grumbled an up-islander).

Atkins decided the turnout represented "a commitment" on the part of summer residents to island health services.And Rose Styron, for one, was living testimony of that.

"I'd do anything for the hospital, she said, describing how she had been delivered of a baby there 20 years ago, sandwiched by the doctor between somebody's broken arm and somebody else's imbedded fish hook. "The doctor asked me, 'Oh say, you don't want an anesthetist, do you?' and I said 'Oh, heaven's no, I wouldn't think of it.'"

Hundreds of broken arms, fish hooks and newborn babies later, the hospital has grown to the point of a $189,000 annual deficit in operating funds. Some year-islanders are agitating for Duke County Commissioners to levy a head-tax at ports of entry.

Georgia Ireland of Chilmark, executive director of the Martha's Vineyard Community Services, said she thought up the idea of a benefit a year ago and decided to tell artist Raymond Ellis about her "new way" to raise funds.

"Since the Ellises knew the Ikards, I said why don't you all get together and plan something. The island doesn't have a society, you know. It's just a place where people relate to other people."

Well, sort of relate.

Down-islanders, who live on the eastern half of this 20-mile long stretch of moors, pine woods, ponds, inlets, harbors and white sand beaches, are gregarious and social, chintz-tidy behind the white-clapboard and weathered-grey shingle houses whaling captains once built.

Up-islanders closet themselves in nooks and crannies at the western end, keeping their distance through "intellectual" pursuits and priorities. When typewriters aren't clicking, tennis balls zing across private nets or fishing lines are experimentally lightened to surf-cast for striped bass.

Edgartown, at the eastern end, is "yachty, preppy, waspy and Yankee," as a relative newcomer put it, while Chilmark, at the western end, "is more ethnically diverse, with families who like to fish, beach and skinny-dip if the mood strikes them.

There is no resemblance whatsoever to Nantucket, writes Francine du Plessix Gray in Vogue: "For Nantucket's celebration of the nuclear family, its narcissistic severance from all political concerns, its nostalgia for virgin nature and its striking fixation on gourmet food have made it a more prophetic microcosm of our late '70s hedonism than any community I know.")

Occasionally, some see the folly of their ways. When Lillian Hellman moved from Chilmark (too chummy) to Vineyard Haven, island eyebrows rose. And when Art Buchwald migrated to Vineyard Haven after several years of renting elsewhere, one woman asked how someone of his standing could live in such a place.

"It's so second-class," she told him.

Vineyard conversation, like its social pace, is demanding. "Rejuvenating," says Rose Styron, a published poet in addition to her work for Amnesty International.

The witching hour came earlier for some dancers than for others. Vance Packard's efforts at persuading the captain of the Chappaquiddick ferry to delay the night's last run had been unsuccessful.

Hearing that the Packards and the Duttons were leaving so that they wouldn't miss the ferry, Henry Beetle Hough broke into a sly grin.

"They could swim it in just a few minutes," he noted. "It's been done before."