There could hardly have been a more masculine scene in Washington yesterday than at the Shoreham Hotel. Hundreds of delegates to the Vietnam Veterans of America convention filled the lobby and corridors, a panorama of men draped in combat memorabilia and camouflage fatigues, of men with shins shiny with scar tissue, of common men sharing uncommon experiences.

But this fraternal society turned not to one of its brothers, but to a sister veteran, electing Mary R. Stout, 43, of Arlington as the second president of the VVA Saturday.

Stout, a former lieutenant in the Army Nurse Corps who spent a year working in a surgical hospital in Vietnam, is believed to be the first woman to lead a national veterans organization. She joined the VVA's Columbus, Ohio, chapter in 1981 after having symptoms of post-traumatic stress syndrome, an affliction relatively common among veterans of the United States' longest war.

Then, she was looking for emotional support and the "immediate trust" of kindred spirits. Now, she is the principal spokeswoman for the only federally chartered group devoted exclusively to Vietnam veterans and their families.

A sober, intense woman who spent much of yesterday fielding questions from reporters and taking advice from well-wishers, Stout was not eager to discuss the significance of a woman's elevation to the head of a veterans group.

Yes, she said vaguely, she brings a "certain sensitivity" to the job. After all, she added, there are 1.1 million female veterans in this country and "veteran is not a male word." However, she said, the VVA's agenda -- compensation for the victims of Agent Orange, judicial review of decisions by the Veterans Administration, a full accounting of Americans who may remain in Indochina, and a shakeup in the VA bureaucracy -- would not be much altered.

But others, including VVA founder and outgoing president Robert O. Muller, saw Stout's election by a 3-to-1 ratio as an important symbolic departure from the traditional all-male hues of other large veterans groups.

"The fact that the delegates elected a woman is one of the clearest statements you can imagine that Vietnam veterans are a totally different generation of veteran -- much more progressive in thinking and not as sexist, racist and reactionary" as other groups, said Muller, a paraplegic who is leaving the presidency to concentrate on aid to the people of Indochina. "It's a helluva statement."

Although Stout does not seem to possess the same firebrand charisma that made Muller the nation's leading advocate for Vietnam veterans, her elevation signals a continuation of the VVA's strategy of being divorced from the traditionally cozy ties binding the Veterans Administration, the Veterans of Foreign Wars and the House Armed Services Committee -- in veterans' parlance, the "iron triangle," according to VVA delegates.

Several veterans said that Stout's election to lead VVA's 30,000 members reflects the maturation of the organization, which Muller founded in 1978 with meager finances and stubborn ambition. Muller, they said, was the dynamic leader who formed VVA from scratch; Stout will be the stable administrator and steward.

Stout went to Vietnam in November 1966 and spent a year working in a hospital intensive care unit and recovery room. On returning home, she put the war largely out of her mind until 1981 when her husband, a career soldier, was sent to Korea.

Under pressure to make ends meet and care for her three daughters, Stout found herself remembering her year in Vietnam, and that led her to the local chapter of VVA. "Just being with other Vietnam vets, you realize you're accepted . . . . There's an almost immediate trust."

In 1983, she moved to Arlington to work in the VVA's downtown Washington headquarters as membership director, and subsequently she moved through the group's ranks to national secretary. Her election as president was widely expected, and the convention floor was a sea of green "Mary" buttons pinned to delegates' lapels.

In an interview yesterday, Stout said: "The leadership, the future of this country is going to come from Vietnam vets who are committed to never sending our men and women to war without the total commitment of their country."