CARMEN VIVANCO VEGA is the kind of quiet woman best imagined tending to grandchildren or talking over old times with family. But much of Mrs. Vivanco's family is gone, lost in the nightmarish oppression of Chile's military dictatorship in the 1970s under Gen. Augusto Pinochet Ugarte. The Santiago resident lost five family members to the military's secret war on Chileans it thought might be dangerous: her husband, her only son, a brother, his wife, and a cousin. They were ''disappeared'' (taken away by police and presumed killed) in early August 1976. Vivanco never knew why, and she has had no information since. Now, she spends most days in a drafty downtown Santiago office, a volunteer for a support and lobbying organization for the families of Chile's thousands of detained and disappeared. Her simple yet seemingly impossible goals: justice for the families of the disappeared and reconciliation for all Chileans. ''For the families, we want to know what became of our loved ones, we want peace,'' Vivanco says. ''For Chile, we want to end this ordeal, but with truth and justice. No country can build its future on a past of so many forgotten or unclarified crimes.'' How to make reconciliation possible, how to serve justice - and whether or not justice is even possible - are questions that continue to perplex not just Chile, but Argentina, Peru, Brazil, and other countries facing up to a past of state repression, torture, and death. In a South America where military dictatorships have given way to democracy and efforts to improve human and political rights, the not-so-distant years when fears of communist subversion and other leftist movements drove many governments to unfettered repression can seem of another era. The temptation in the face of expanding evidence of thousands of civilian disappeared in the dictatorships' ''dirty wars'' can be simply to forget the past. ''I recognize that past actions dealt these people a terrible blow, but they should move on from the past, just as the rest of the country has,'' says one Argentine moving hurriedly through Buenos Aires' Plaza de Mayo. Here, every Thursday for 18 years, the mothers of Argentina's disappeared have marched for justice. But events over recent months have proved that an ugly past, especially when only partially unearthed, is not easily dismissed. r Since May, Argentina has been rocked by successive confessions from former military leaders about the detainment and disappearance of 10,000 to 30,000 Argentines during the country's military rule from 1976 to '83. The most chilling revelations told of weekly military flights over the Atlantic from which drugged political prisoners were dumped into the sea. In other cases, detained pregnant women were kept alive long enough to bring their babies to term - so that childless military families could adopt them. r In Chile, tensions resulting from the unserved sentence of a former military leader for his role in a 1976 political assassination have led to months of protests and political discussions over how to ''close out'' the torturous past. On Aug. 21, President Eduardo Frei Ruiz-Tagle announced a series of judicial proposals and constitutional changes designed to help establish the truth about the country's disappeared, and to remove the last vestiges of the 17-year military dictatorship under General Pinochet that impede Chile's democratic rule. President Frei proposed offering secrecy and impunity to officers who come forward with information about the disappeared. But, General Pinochet, who remains Army commander, has starkly rebuffed that plan. In addition, opposition from far-right legislators all but guaranteed the initiative's failure. r In Peru, newly reelected President Alberto Fujimori caused an uproar in June when he signed into law an amnesty of all human rights abuses committed by the military in 15 years of antiguerrilla warfare. r And in Brazil last month, the government is drawing fire from human rights organizations for its plan to compensate the families of 136 recognized disappeared citizens. (See story at right.) What these events suggest is that none of these countries has found the magic formula for delivering justice and closing out the past. Hopefully named ''final point'' laws designed to address once and for all such dictatorship-related issues as truth, compensation, and reconciliation, have not freed these countries from the past. Argentina's approach was to convict and imprison - and later pardon - the leaders of the country's military junta. Lower officers could not be tried for any regime-related crime on the assumption that they were acting on policy set at the top. In Chile, on the other hand, an amnesty law passed by decree under Pinochet covers rights abuses only for the period from 1973 to '78. But until the May conviction of two officers in a 1976 political assassination, no aspect of the military rule had been condemned, and Pinochet remains a key force in the country's incomplete democratic transition. Historical differences are one explanation for the various approaches. For a civilian government to condemn military rulers was easier in Argentina, given the disastrous shape the government left the country after the Falklands war against Britain. The picture was different in Chile, where much of the public still credits Pinochet with having brought the country back from the economic ruin of Socialist President Salvador Allende Gossens. In fact, those who support the military governments often accuse the public and civilian politicians of applying today's standards of human rights to events of a decade ago or more. ''The military government ended the very real threat of civil war in Chile, and it succeeded in the country's social and economic transformation that most Chileans praise today,'' says Jorge Ballerino, a retired Army officer in Santiago and a former minister in Chile's transition team back to civilian rule. In Argentina, sympathizers of the military regime say that the country was at war against subversion, and that in warfare ugly things happen. Others say that's a dangerous revision of history that only makes reconciliation more difficult. ''To say that Argentina was at war during those years or that, barring one province, there was an organized and threatening guerrilla [insurgency] is just absurd,'' says Emilio Mignone, a leading Buenos Aires human rights lawyer and authority on rights abuses during the military regime. Mr. Mignone's own daughter was taken from the family's apartment by police ''for questioning'' one morning in May 1976 and never heard from again. ''Her most suspicious crime was being an altruistic college student who volunteered a few hours a week with a religious group working in a shanty town,'' says Mr. Mignone. ''Is that a threat against the state?'' Observers most closely aligned with the military's point of view tend to promote the idea of moving forward. Rosendo Fraga, an Argentine defense specialist, says it is the very steps Argentina has taken - including a pardon of military rulers - that have allowed more of the truth about the past to come out, in the form of various officers' ''confessions.'' But he questions the purpose of such actions. ''There is no objective opinion possible on the merit of these revelations,'' he says. ''For some they are useful because they educate society about something that should never be repeated, but for others they are negative because they divide us once again over the past and turn the country away from looking forward.'' The families of the disappeared and the groups supporting them are themselves divided. In Brazil, the private group Torture - Never Again, which represents victims' families, is not pushing for legal action against the dictatorship's leaders, but does want torturers and murderers identified and banned from holding public office. Hebe de Bonafini, internationally known leader of Argentina's Mothers of the Plaza de Mayo, goes further. ''We already know the truth: Our children died by torture,'' she says. ''What we want is prison for the assassins - no reconciliation, no remains, no investigations, and no money.'' Viviana Diaz, president of Santiago's Association of the Families of the Detained and Disappeared, says for her group ''justice'' means prison sentences for authorities who participated in torture and murder. But she says her first concern is ''establishing the truth about every single detained and disappeared'' person - and that is why December's approach has her worried. At the end of the year, Chile's National Corporation for Reparation and Reconciliation - the office responsible for investigating the cases of the disappeared and for filing families' reparation claims - is to be phased out. But Executive Secretary Andres Dominguez Vial recognizes the corporation's work is far from done: Of the 3,197 recognized deaths and disappearances, about 1,000 have yet to be cleared up, he says. IN the interest of both the families' peace of mind and Chile's ability to move forward, says Mr. Dominguez, the country should approve an amnesty similar to Mr. Frei's for persons who come forward with information. ''The purpose would be to further a very difficult healing process, not to open old wounds with court cases against the guilty,'' he says. Dominguez, who was exiled for six years for his opposition to the military regime and who lost a brother in the repression, says the goal of his work is reconciliation from a political point of view - to guarantee the country's governability and adherence to democratic principles. Personal reconciliation, he says, is not in the government's power, and will take generations. Nor is it realistic for politicians in Chile or elsewhere to promise laws that somehow close the door on the past, he adds. ''To speak of a final point is absurd, history has no final point,'' he says. ''Look at the [Klaus] Barbie case in France and others dating from before and during World War II that resurface today.'' Standing under a folk art embroidery stating, ''The truth is the beginning of justice,'' Dominguez says. ''There are things governments and people can do to clarify the past and establish the facts in the interest of a better future,'' he says. ''But another truth is that 50 years from now we will still view these years as difficult memories.''