The choice of what to wear to a Washington social event is fraught with challenges and hazards. Not to be overdressed, not to be underdressed, not to be identically garbed as another guest--or worse, the hostess--are just a few of the considerations.

For most occasions, such gaffes are hardly serious. But when you are invited to dinner with the editor-in-chief of Vogue magazine, the rules change.

"I figured I could be less conservative than usual," said Nancy Rubin.

"I wore what I thought was the nicest thing I own," said Nancy Rosebush.

"A command performance," said Buffy Cafritz.

For most of the guests at the dinner honoring Grace Mirabella at Saks-Jandel, dressing up was in order.

"My husband wanted me to wear something synthetic. I said--No way!!!" said Rosebush, whose husband, Jim, is a deputy assistant to President Reagan. She was wearing a white jacket by Anna Weatherly.

At dinner, Saks-Jandel president Ernest Marx saluted Vogue as the "arbiter of taste--good, bad or indifferent." Mirabella, taken aback by the toast, countered by suggesting that guests help themselves to the lavish furs on display as a souvenir of the evening.

Mirabella, who was the drawing card for the party given by the store to show off its remodeled fur salon, wore a silver-gray lace gown by Geoffrey Beene, a clear favorite of hers. She also was wearing a necklace by Arthur Koby, which she borrowed from Beene. An antique porcelain doll was attached to the center of the stone choker.

But the lavish furs on display in the store had tough competition from Tuesday night's guests. Samia Farouki, who is partial to Galanos, considered wearing something black "since Vogue magazine says black is the color this season," but opted for a backless Galanos in brown to stand out from the crowd.

And although black was the predominant color for both long and short dresses, Helga Orfila and Vogue executive editor Barbara McKibbin opted for Zoran's pale yellow separates.

"This is the season for sequins by the yard," explained Buffy Cafritz, who demonstrated the point with a new gold-sequined tunic and black skirt by Halston.

Others opted for favorite dresses from earlier seasons. Maureen Bunyan wore a short purple dress from Yves Saint Laurent that was four years old; Pat Schieffer had on a black gown by Beene appliqued with lace and rhinestones.

Saks-Jandel vice president Val Cook, who guides many stylish Washington women on what to buy and wear, was on the phone to New York several times before she decided to wear a pewter-and-putty-colored sequined tunic and taffeta skirt by Bill Blass. "I had only one goal--not to wear the same dress as the guest of honor," said Cook.