Five years after the last Americans from Saigon and the arrival of victorious Viet Cong forces, the United States and Vietnam have launched a process that could lead to the orderly and direct immigration of thousands of Vietnamese to this country.

Ever since Vietnamese began to flee in huge numbers in rickety boats and by hazardous journeys overland to Thailand, American and international relief officials, as well as the countries where the so-called "boat people" have landed, have sought a formula by which Vietnamese who wanted to leave could do so without risking their lives.

More than half a million have fled by this perilous route and only about 2,600 by direct departures under U.N. auspices. Only a few hundred in this latter category have come to the United States.

Early this week, however, an American citizen began work as an employe of the United Nations High Commissioner for Refugees interviewing 1,758 Vietnamese in Ho Chi Minh City (formerly Saigon) in the final step of a long and arduous negotiating process that could lead to Vietnamese leaving for the United States in a steady flow within four to six weeks.

Many of the first to leave would be classified as immigrants and would not come in under U.S. refugee quotas. They are relatives of Vietnamese already living in the United States or former employes of the United States who were caught in Saigon after the abrupt U.S. pullout.

Negotiations leading to this orderly departure program, as it is called, have been conducted through the United Nations, reflecting the mutual suspicions that are the legacy of the Vietnam War on both sides.

Nevertheless, U.S. officials have used words such as "hopeful" and "breakthrough" in describing the Vietnamese decision to allow the American, Thomas Malia, to begin the interviews. The final stages of the process would involve U.S. consular officials from Bangkok going to Ho Chi Minh City for brief, direct interviews before the prospective immigrants boarded the weekly Air France flight to Bangkok.

Except for a few U.S. officials who have gone to Hanoi in connection with repatriation of the bodies of U.S. servicemen or for discussions on American missing-in-action, the consular officials will be the first U.S. officials to conduct business on Vietnamese soil since the end of the war.

Why the Vietnamese chose now to allow the program to proceed is not at all clear, although analysts in Washington and Bangkok noted that this decision and an invitation to discuss the question of the missing-in-action came initially in September following serious setbacks in Hanoi in the United Nations about Vietnamese presence in Cambodia and rebuffs by its Southeast Asian neighbors on the same issue.

"This is the most encouraging development to date," Shep Lowman of the State Department's Office of Refugee Affairs said yesterday. "We can't be 100 percent sure this will actually result in a flow of people out of Vietnam, but this is encouraging and we are hopeful . . . ."

Beyond the initial list of 1,758 on whom the Vietnamese and U.S. are in preliminary agreement, there are thousands of others designated by one country or the other as potential immigrants. The United States, according to the State Department, has given Hanoi a list with more than 10,000 names and Vietnam has sent Washington a list of 30,000 names.

While the negotiating process on setting up the program has been going on for more than a year, and while the negotiating breakthrough came in September, it was only early this week that Malia was allowed to go to Ho Chi Minh City to begin his work.

Not all the delays have been on the Vietnamese side, however, according to diplomatic sources in Bangkok. While the United States has been eager to develop some type of program that would avoid the horrendous scenes of Vietnamese fleeing in rickety boats, it also demanded lengthy interviews, verification of personal histories and documents, medical exams and other paper work before an applicant could leave Vietnam for the United States. All of this is mandated under U.S. immigration laws.

The Vietnamese, still highly suspicious of the United States, would have none of it. Not only did they oppose the stationing of four American consular officers on Vietnamese soil to do the interviewing, as Washington proposed, Hanoi apparently felt the orderly departure program should simply be the swift transfer of a huge number of undesirable citizens to foreign countries with minimum of bureaucratic fuss.

Moreover, boat people picked up in the South China Sea by U.S. Naval vessels were guaranteed resettlement without any screening by U.S. officials. v

"Why did they not put consular officiers on the 7th fleet?" asked a Vietnamese diplomat based in Bangkok.

Malai's presence appears to satisfy both sides. He is not a U.S. official, but he is familiar with U.S. immigration and refugee law. He will interview those on the list and make a recommendation to U.S. officials in Bangkok.

"The obvious arrangement was to have someone interview them whose opinion the U.S. would accept," said one refugee worker in Bangkok.

Meanwhile, the flow of Vietnamese and other Indochinese fleeing their countries by boat or land continues. More than 7,400 reached camps throughout Southeast Asia during September, the last month for which statistics are available, while some 17,000 left for resettlement, 12,000 of those to the United States. The total remaining in the camps, however, is 207,214.