Washington — The Reagan administration is walking a delicate political and diplomatic line regarding the nearly 2,500 Americans still unaccounted for from the Vietnam war. In Dallas this week, veteran's organizations are charging that the Defense Department and United States intelligence agencies have covered up information showing that many Americans still are being held in Southeast Asia, more than 11 years after hundreds of American prisoners of war (POWs) were released.
Also this week, US officials return from a ''technical meeting'' in Hanoi - the first such session in more than a year - at which new data on unaccounted-for Americans may be provided.
The administration finds criticism of its policies on POWs from veterans and conservatives ironic. Vietnam POWs have been a subject of strong Reagan concern since his early days as governor of California. And Reagan pledged to achieve ''the fullest possible accounting'' when the remains of a soldier from Vietnam were interned at the Tomb of the Unknowns this spring.
Still, the administration is not hopeful that live US servicemen will return, according to American military officials. And it is taking a firm but realistic position lauded even by Democrats who oppose its foreign policy.
The US government presumes that only one of the 2,483 missing is living. This is Air Force Col. Charles Shelton, known to have been in good shape after he was shot down over Laos. Air Force Secretary Verne Orr now is deciding whether to change Colonel Shelton's status.
In all other cases, a ''presumptive finding of death'' has been issued based on the weight of intelligence data. In about 1,150 cases, fellow soldiers were sure they saw the man killed but were unable to retrieve the remains after the battle. In many others, damaged aircraft are presumed to have crashed into the sea.
Based on information made public in Gen. William Westmoreland's lawsuit against the CBS television network, however, Washington charges that Vietnam has much more information on the fate of many of the missing.
''We're relatively convinced that they have extremely good records,'' says US Army Lt. Col. J.R. Shields Jr. ''Records of shoot-downs, records of captures, records of internments, records of burials.''
There also have been more than 2,600 reports of ''live sightings'' of Americans in Southeast Asia, 642 firsthand and some as recently as 1982. These come mostly from refugees, some of whom are seeking asylum in the United States.
The National Vietnam Veterans Coalition said in Dallas this week that it had obtained data proving that 22 Americans were being held in Laos.
The Defense Intelligence Agency (DIA) and Joint Casualty Resolution Center in Honolulu are actively investigating about 85 of the most recent live-sighting reports. But US officials have disposed of most of the refugee reports, in some cases using intelligence sources (as one officer puts it) ''that only the CIA and God know about.''
US officials privately doubt that many (if any) Americans still are being held captive in Southeast Asia. And they are reluctant to discuss the acknowledged possibility that some soldiers deserted or chose to stay once captured. They know that at least a handful stayed behind voluntarily and now have families in Southeast Asia.
The Vietnamese government has been more forthcoming on the subject in recent months. In February, Assistant Defense Secretary Richard L. Armitage led the highest-level US government group to visit Vietnam since hostilities involving US troops ceased.
''The Vietnamese agreed to accelerate their search efforts, with initial focus on the most accessible cases of our missing men and the cases of those Americans whom the Vietnamese said died in captivity,'' Mr. Armitage told a congressional panel recently. In July, Hanoi released the remains of six more US servicemen, bringing the total to 94.
Armitage also testified that a ''cottage industry'' has developed among refugees offering purported aircraft crash site data and even human remains to US investigators. ''In most cases,'' he said, ''The remains, for which the refugee may have paid, are determined to be animal bones or of Southeast Asia mongoloid origin.''
''We have not yet been able to prove or disprove that there are any Americans being detained against their will,'' the DIA director, Lt. Gen. James A. Williams, told the same House hearing August 9. But, he added, ''based on the number of reports received concerning live Americans in captivity in Southeast Asia, we are proceeding under the assumption that some Americans are still held, and we will always predicate our actions on that basis.''