The Band Corp., which counseled the American defense establishment on the Vietnam war, concludes in a new study that South Vietnam's exiled leaders regard the United States as a major culprit in their defeat.

The study was based on interviews with 27 former high-ranking South Vietnamese officers and civilians. Their view was that the United States played the role of a "gigantic but somewhat blind and often oppressive 'super-ally' who did not clearly understand the nature of the war." But they also concluded that the top echelons of South Vietnamese leadership were riddled with corruption, incompetence and cowardice.

Rand, the premier "think tank" for the U.S. Air Force throughout the war, received $226 million in Department of Defense contracts during the 12-year period from 1964 through 1975, when Saigon fell. And South Vietnamese leaders during the war denounced printed reports on corruption as being defeatist or communist propaganda.

With the advantages of hindsight, political asylum in the United States and the confidentiality extended to those who requested it, the former South Vietnamese leaders were candid and even matter-of-fact about their own corruption.

"... There was not one highranking person in the Saigon government who was not accused by at least some of the respondents as having participated in the corruption and profited from it," said the report, which was prepared for the Department of Defense historian.

The corruption mentioned came chiefly in four forms: racketeering in scarce and often vital goods, bribery of officials, selling of jobs and appointments and collection of army pay from "ghost soldiers" and "rollcall soldiers."

"A "ghost soldier" was one who had been killed in combat but was still carried on the army rolls. "Rollcall soldiers" were deserters.

The most eloquent indictment of South Vietnamese corruption came from an unidentified "top commander," who said:

"Corruption always engenders social injustice. In Vietnam, a country at war, social injustice was more striking than in any other country. Corruption had created a small elite which held all of the power and wealth, and a majority of middleclass people and peasants who became poorer and poorer and who suffered all the sacrifices.

"It was these people who paid the taxes to the government, the bribes to the police, who had to buy fertilizer at exorbitant prices and to sell their rice at a price set by the government, and it was also these people who sent their sons to fight government, and it was also these people who sent their sons to fight government, and it was also these and die for the country while high people sent theirs abroad.

"An army doctor once told me that he was disheartened to see that all the wounded, all the amputees who crowded his hospital came from the lower class, from the peasants' families, and that they had suffered and sacrificed for a small class of corrupt elite."

The three authors of the report -- Stephen T. Hosmer, Konrad Kellen and Brian M. Jenkins -- say in a brief preface that they tried to summarize what the respondents said without evaluating their statements. The interviews have been withheld by Rand to insure the promised confidentality.

In some cases, however, former South Vietnamese officials, like one-time premier Air Marshal Nguyen Cao Ky and former South Vietnamese ambassador Bui Diem, have made on-the-record statements.

The authors say that most of those interviewed believed that an effective defense could have been continued in South Vietnam except for the reduction of U.S. aid after the Paris accords in 1973.

"Most also believed that the reintervention of U.S. airpower, especially B52s, would have prevented defeat in 1975," the report said. "None would contend that continued aid or renewed intervention would have ended the war or brought victory -- significantly, the word 'victory' does not appear in the interviews."

According to the report, the South Vietnamese leaders were unanimous in regarding the Paris accords of 1973 as "a turning point for the worse." But South Vietnamese President Nguyen Van Thieu and others in the government nevertheless maintained "an unshakable conviction that the United States would come to South Vietnam's aid in case of real trouble."

This conviction was based, at least in part, the report says, on conversations that Thieu had with President Nixon at San Clemente on April 2-3, 1973. Bui Diem, who was present at the meetings, said Nixon told Thieu privately: "The U.S. will meet all contingencies in case the [Paris] agreement is violated," and "You can count on us."

"So pleased and relieved was Thieu with these results of the San Clemente meetings that, again accoring to Bui Diem, he had champagne broken out to celebrate as soon as his plane was in the air," the report said.

The report was sharply critical of Thieu, a criticism accented by Thieu's refusal to be interviewed and by several comments of Air Marshal Ky, a Thieu political opponent.

Ky, who operates a liquor store in Newport Beach, Calif., says at one point that Thieu feared assassination and slept in a different room every night. Apparently Thieu's fears of at least being deposed were justified, for Ky acknowledges plotting with other Vietnamese to replace Thieu -- a move he says was discouraged by the Americans.

Ky is particularly outspoken in the report about the dependent relationship which grew up between U.S. military advisers and their South Vietnamese counterparts.

"After a few years there is some sort of Mafia established between American advisers and the Vietnamese commanding officers because, you know, they need each other to get promotions, they need good records and recommendations," Ky said.

In answering questions about the report, Rand officials deemphasized the research organization's involvement in Vietnam war strategy. They said that less than 10 percent of the $226 million in defense contracts it received during the war was spent on projects directly relating to the war. And they pointed out that one of the report's authors, Kellen, signed a letter that appeared in The Washington Post on Oct. 12, 1969, calling for complete withdrawal from Vietnam within a year.

Five other Rand associates, including Daniel Ellsberg, signed that letter. A Kellen report in 1969, classified until after the war, suggested that the Vietcong were "not capable of collapse and surrender."