The white-gloved waiter tapped gently on the lady's elbow. "The other side, madam," he said discreetly, directing her to stand at her husband's left as she went through the receiving line.

It would never do, of course, when being introduced to Secretary of State Vance and his wife, who received the diplomatic corps of Washington last night at a black-tie dinner held in the Benjamin Franklin Room at the State Department. As one would expect, everything was comme il faut .

"Oh dear, they've given drinks before the receiving line," said another distinguished diplomat. "Well, we'll just have to leave them here," said his wife." "Or we could finish them before we go through." No, said her husband, then they would be late. They left their white wines behind.

While in real life, diplomats are often regular people-the sort who have children with braces, and wear frayed collars, blisters, clothing they fear may be improper-last night some 300 of them moved through the pageant of propriety outlined like a familiar yet endearing stage script.

One is introduced by an "escort officer," and one receives a handwritten engraved card indicating the table to which on is to proceed when told that cocktail hour is over. At the table one finds a handwritten engraved placecard, introduces oneself to one's dinner partners, and stares at the array of silverware and wine glasses.

It was recalled last night, by several who should know, that when Henry Kissinger was Secretary of State he discontinued the annual event. But Vance has reinstituted the black-tie function, and most of the guests seemed to think it was an excellent idea.

"You get a chance to see your colleagues," said one ambassador. "Ambassadors whisper things into each others' ears and you remember what they said. Certainly any ambassador who is able would come-I would be very bad manners if they didn't.

At table, partners were mixed by continent and spouse, so that different countries had a chance to sit together.

Dinner began with Mushroom Soup, as the string ensemble of the U.S. Army Band played away in a corner of the room.

Next came Salmon en Croute, which some people embrassingly took to be a fish course and-horrors-they used the wrong fork . It became clear, when a veritable chorus line of black-coated waiters emerged from one corner carrying silver bowls of salad, that Salmon en Croute had actually been the main course. Then they brought out glass bowls containing what appeared to be water with two rose petals floating on top.

At one table, several people waited expectantly for someone in the know to show them what to do with the things. They soon learned to remove the small bowl of water to the left of one's plate, remove the doily, and prepare to be served Grand Marnier Souffle.

Secretary Vance, in his after-dinner toast, said the guests "reflect what is the finest in the diplomatic tradition." Not only were they skilled, said Vance, but they "share a broader vision of peace and humanity."

The return toast was made by the dean of the diplomatic corps, Dr. Guillermo Sevilla-Sacasa. He thanked the Vances for their hospitality and spoke flowingly of Benjamin Franklin, Thomas, Jefferson, "the beautiful and charming women" who were at the dinner, and proposed a toast "to the president."

After the Strolling Strings of the U.S. Army Band strolled through, singer Roberta Peters, dressed in a scarlet straples gown, provided entertainment. She sang Mozart, Lehar and some Victor Herbert, including 'Kiss Me Again." CAPTION: Pictures, Cyrus Vance greets a predecessor, William Rogers, as Kit Dobell talks with adele Rogers, by Linda Wheeler-The Washington Post