Frankie Valli and the Four Seasons sang "Walk Like a Man" and other pop anthems of the 1960s on the Mall yesterday before thousands of Vietnam veterans, many in their battle greens and others in the pin-striped grays and multicolors of civilian life.

The concert highlighted a day of happy celebration and sad reminders for the veterans who are in Washington for today's Veterans Day ceremonies at the Vietnam Veterans Memorial.

With the U.S. Capitol looming in the background, entertainers joined onstage with Medal of Honor winners and several Gold Star Mothersfrom the Vietnam war in an unabashed demonstration of patriotic feeling.

At noon, four F-4 Phantom jets and 10 Huey helicopters flew over the crowd -- estimated by the National Park Service to number more than 5,000 -- to a chorus of cheers. "Hearing those helicopters causes a chemical reaction in some of us," said emcee Larry Matthews, a WMAL news reporter.

Tom Dineen of Annapolis, a former airborne Ranger in Vietnam standing on the fringes of the crowd, looked out on the scene and retraced the troubled history of the veterans who bear the stigma of having fought America's most controversial war.

"I think the veterans are feeling better," he said. "Veterans are a lot more open with each other now. The people are treating them better.

"I think the wall the granite, V-shaped Vietnam Veterans Memorial has been a big part of that."

Many of the returning veterans were meeting for the first time since the war, while others had returned to Washington after attending the memorial's dedication two years ago.

The weekend activities will climax today at 3:30 p.m. near the Vietnam Veterans Memorial, with President Reagan scheduled to speak at a ceremony dedicating the "Three Fightingmen" statue and turning the memorial over to the federal government.

"A lot of coming together and healing occurred after 1982. We're trying to enhance that," said George Sullivan, organizer of the weekend "Salute II" activities.

"Nineteen eighty-two was the welcome home, but the emphasis this time is on unity with all veterans. Not just Vietnam veterans, but veterans from Korea, World War II and World War I."

Contrasting this year's events with those of 1982, Sullivan observed: "Last time there was a lot more emotion. This time there's a lot more positive, up feeling."

Some veterans, however, repeated the sentiment, often heard two years ago, that Vietnam veterans never have gotten their due from the American public.

"It's getting popular to be a Vietnam veteran now," said 42-year-old Ralph Sambuchi of Maynard, Mass., who wears a black patch over the eye he lost in Vietnam.

"I tell people, 'Get the hell away from me. Where were you 15 years ago?' "

William Sauter, 37, a high school teacher from New Hampshire who served with the Marines in Vietnam in 1967, said he was here for the dedication of the memorial in 1982 and returned for the ceremonies this weekend.

"I still don't see the American public out here," he said. "The Redskins or a basketball team would get a better reception. I'm just a member of a forgotten generation."

With the concert under way near the Capitol, the largest throng in two years gathered at the Vietnam Veterans Memorial at the other end of the mall, according to Park Service volunteers there.

Families laid wreathes and flowers at the granite wall, while veterans touched the names of lost comrades.

Shortly before 3 p.m., a hush came over the crowd as a bugler sounded Taps at the vertex of the wall. He was framed on either side by men carrying the black-and-gray MIA flag and the blue-and-orange flag of VFW Post 4796 of Wickliffe, Ohio. Officials said the post was the first all-Vietnam veterans post admitted to the VFW.

John D. Stefanik, 36, a former Green Beret who was holding the MIA flag, spent 2 1/2 hours searching among the 58,022 names on the wall and touching the individual names of 22 friends killed during the 4 1/2 years he was in Vietnam.

"My hand just shook as I touched each name," he said. "Finally, somebody paid respect to them for the loss of their lives. The whole United States does not know about what they did individually. I do."

In a panel discussion yesterday morning, sponsored by the Vietnam Veterans of America, experts on post-traumatic stress disorder told a group of veterans and their families that PTSD is the modern version of the "battle fatigue" of World War II and the "shell shock" of World War I.

Eric Gerdeman, clinical director of a veterans outreach center in Silver Spring, said veterans must learn to accept that they are "real survivors" and stop wishing "someone would give me a parade."

The numbers of those suffering the symptoms associated with PTSD, psychologists say, are not expected to peak until 1985 or 1986.

Sullivan said that he conceived had the Salute II event to hasten veterans' resolution of the war-related troubles. But he said yesterday that he has had difficuly raising money and had to mortgage his house to help pay for this weekend's activities.

Hats were passed at the concert to help Sullivan defray his expenses, but he said that, so far, only about a third of the $100,000 cost of the weekend events has been raised.

Valli performed for free.

"This is a kind of payback for me," he said. "A lot of those guys bought a lot of my records."

Launching into a series of hits that spanned the 1960s, Valli played to a crowd that seemed content to return to a time in which many of them remain firmly rooted.

Whether in groups of old war buddies in green fatigues or alone in silent communion with the memorial,, many veterans who returned to Washington yesterday still find their lives shaped by the Vietnam experience.

"It's never been resolved," said the wife of a veteran who had traveled here from Massachusetts with her husband. "He still lives it over and over again.

"Maybe seeing the wall and the names will help him get rid, a little bit, of what he's been carrying inside him for 15 years now."