Proclaiming an end to years of government neglect, President Reagan yesterday hailed the "steadfast, wondrous" courage of more than 500 relatives of soldiers missing in Vietnam.

Appearing before a special meeting of the National League of Families of American Prisoners and Missing in Southeast Asia, Reagan told the crowded ballroom at the Hyatt Hotel in Crystal City: "Your long vigil is over. Your government is attentive and the intelligence assets of the United States are fully focused on this issue."

As if to underscore his assertions, the president was flanked by the secretary of defense, his national security adviser and the chairman of the joint chiefs of staff. Draped behind Reagan was the stark black banner of a once-unpopular movement: The POW-MIA flag sewn by the wife of a serviceman listed as "missing in action," with the silhouette of a soldier on it and the words "You are not forgotten."

Vowing that the United States would take "decisive action" on any verifiable reports of American soliders still held captive in Vietnam, Reagan said: "I call upon the government in Hanoi to honor their pledge to the American people on the POW-MIA issue. Not for me, not for our government, but for our missing men and those of you who did nothing to deserve this terrible emotional ordeal."

Interrupted several times by applause, Reagan said that the MIA families had been "unfairly and cruelly branded" by their own government, and he called the families gathered in the Regency room "heroes who kept a vigil of unprecedented faith and devotion."

It was a kind of political homecoming for the family members of the 2,494 Americans still unaccounted for in Southeast Asia. Until Indochinese refugees began bringing back reports of sightings of American prisoners, the league had been written off by government officials as a collection of emotional relatives. League members, as Reagan noted, had to demonstrate in front of the White House in 1974 and 1975.

"I've been waiting 15 years to hear something like that," said Jennie Page of Deerfield N.H., whose only son, Air Force Maj. Albert Page, disappeared over the Gulf of Tonkin 15 years ago. "He's given us hope. I'd just like to know where my son is. Whether he's dead or alive, he has to be somewhere."

But some families harbor cynicism and resentment for the way they were treated by their government. League President George L. Brooks drew loud applause Thursday when he recalled the official accounts the State Department gave many families about their missing servicemen.

"There's no nicer way to put it," Brooks said. "A lie is a lie."

The two days of this special meeting of MIA-POW families married Hollywood and Washington in a way that made Reagan's appearance all the more appropriate.

As part of an effort to publicize the issue, the meeting was convened on the 10th anniversary of the Paris Peace Accords and coincided with the release of 19 public service television spots, featuring Bob Hope, William Shatner and other celebrities urging the public to "ask for an accounting."

The media campaign was developed by entertainer Fred Travalena, whose role drew a grateful acknowledgement from Reagan. Travelena, who gave the league $35,000 he had won on a quiz show called Celebrity Bullseye, said that he got involved when he met league executive director Ann Mills Griffiths at a concert for the returned hostages.

"Ann said, 2,500 men are still missing," Travalena related. "I said, yeah. She said, 2,500 men. I said, sure. She said, 2,500! I said send me something. She sent me all this testimony, and I freaked."

Throughout the meeting it was the valor of Vietnam, not the ignominy, that was remembered. A hush fell over the crowd when Reagan, at the conclusion of his speech, described a reception for returned prisoners of war when he was governor of California.

"I asked Nancy 'Where did we find such men?' " he said. "We knew the answer right away. Where we have always found them: On the farms, in the streets, in the cities. They're the product of the greatest, freest system man has ever known."

Cdr. Ralph Gaither, who was a prisoner for more than seven years and sits on the board of the league, presented Reagan with a hand-carved plaque showing the same silhouetted soldier that was on the black banner behind the podium. The president and the ex-POW embraced.