Ten years after the fall of Saigon, an event which was supposed to have unleashed the red tide of communism in the region, much of East Asia is more politically stable and economically vibrant than at any time in its history.

Before and during the Vietnam years, it was the specter of a united Communist front -- China, the Soviet Union and North Vietnam -- expanding throughout Asia that appeared to underlie many of the decisions committing the United States in Indochina.

A decade after the Americans left, China is no longer the enemy. It is now more at odds with Vietnam than any other country in the world. Its current political and economic reforms mark a new pragmatic approach with reliance on the West and Japan. Vietnam, meanwhile, isolated and embattled with China, is the odd man out.

Despite a vast and still expanding military presence in Asia and the Pacific, the Soviet Union has garnered few tangible political or economic gains from its deployment of military power in the past decade or so. Along with Vietnam and North Korea, the Soviet Union remains almost totally excluded from the economic boom in the region. Its relations with Japan remain at a low ebb and it is too early to tell whether the new leaders in the Kremlin will be able to overcome the major obstacles with China.

Many Southeast Asian countries still suffer from extensive poverty and ethnic conflicts. There are problems of stability and modernization. But during the past generation, Asia has moved from relative economic deprivation and political confusion to a position of considerable confidence, stability and prosperity, according to Henry Kenny, the author of a recent book on the U.S. role in Vietnam and East Asia.

The region as a whole is seen as one of many independent countries whose interests agree closely with those of the United States, and with which the United States has, in turn, diversified its ties. Instead of a narrow involvement with one country or one part of Asia, as was the case during the occupation of Japan and the wars in Korea and Vietnam, the United States has deepened its ties, politically, militarily and economically, with the many powers in Asia.

"There is an expanding practice of regional consultation and a developing sense of common interest in regional security," wrote Secretary of State George P. Shultz in the current issue of Foreign Affairs. "In this sense, a decade after Vietnam, the United States has more than restored its position in Asia." China's Changed Role

"By any measure, the Asia-Pacific region ranks as the foremost success story for U.S. regional security interests during the past decade," wrote Jonathan Pollack in the current issue of the International Security Yearbook published in cooperation with Georgetown University's Center for Strategic and International Studies.

"Despite the prolonged and ultimately unsuccessful American military involvement in Indochina and intermittent doubts within the region about the credibility of American power, the United States currently enjoys a highly favorable position within East Asia and the Pacific, especially in economic and political terms," according to Pollack, a senior staff member of the political science department at the Rand Corporation, a California-based think tank.

Among the most positive changes in the region in the past decade, according to some analysts, are the changed role of China, in its relations with the United States and Japan, the rise of Japan as Asia's leading economic powerhouse and one that is growing in political influence, and the general economic growth of the region as a whole.

As a result of China's split with the Soviet Union in the late 1960s, "the related improvement in U.S.-Chinese relations promised a political realignment in Asia unparalleled in a generation, . . . renewed U.S. interest in Asia, and formed the background for improved Chinese relations with Japan, Thailand, and other states in the region," wrote Kenny, a senior officer for Asia with the U.S. Arms Control and Disarmament Agency.

"Now China is seen as more independent, more close to the United States and western interests," said Robert Sutter, an Asian affairs specialist with the Congressional Research Service. "China has cut back its role as a source of instability. If you were to do a risk analysis of the region, it is a positive force in the region now."

In the past decade, the nations of the Pacific rim, with Japan in the lead, have replaced Western Europe as the United States' major trading partner. In 1984, U.S. trade with East Asia was about $170 billion, a 34 percent increase over 1983, accounting for 31 percent of total U.S. trade. Trade with Japan alone reached $80 billion last year. By contrast, trade with Western Europe was $144 billion, an increase of 12.5 percent over 1983.

In this season of Vietnam retrospectives, many have argued, most notably Henry Kissinger, that the Vietnam War was a worthwhile cause because it bought time for the potential "dominoes" to strengthen themselves and develop.

Certainly Thailand benefited from a major influx of U.S. cash during the time. But the flaw in that argument, according to some analysts, is the assumption that somehow or other, Vietnam would have been an aggressive power in Southeast Asia.

"The Vietnamese revolution was confined to Indochina," said one high-level administration official. "There is little evidence that Vietnam was supporting insurgents outside Indochina."

The domino theory was not credible, in this view, because it was based on an idea of an irreversible communist expansion. "Nations with strong nationalistic backgrounds don't commit suicide," the official said.

As for Laos and Cambodia, he said, "Laos was never a country and Cambodia was part of the Indochina war." The Vietnamese have long regarded Indochina, including Laos and Cambodia, as their bailiwick. A western diplomat in Bangkok recently said, "What they really want are satellites," like the Soviet Union has.

There is recognition, however, that the wartime failure of U.S. policies in Indochina dramatically demonstrated the limitations of American military power in local Asian conflicts and, according to Tommy Koh, Singapore's ambassador to Washington, galvanized the economies of the noncommunist countries. It also forced them to deal with their internal security problems, resulting in greater Asian self-reliance and self-confidence.Effect on U.S.-China Ties --

It is less clear how the war shaped U.S.-Chinese rapprochement.

"To a degree, it complicated the Chinese freedom of action, but not by much, because the China-U.S. reconciliation was due more to the development of the Sino-Soviet split," said one U.S. official.

By the end of the 1960s, as a result of the Soviet military buildup around China, the Soviet invasion of Czechoslovakia in 1968, the Sino-Soviet border clashes of 1969 and the "Nixon Doctrine" of 1969 calling for reduced direct American military involvement, as well as for its own internal domestic reasons. Balance of Power

"China is fundamental to keeping the balance of power in Asia," said the Congressional Research Service's Sutter. "Japan is important from a strategic and U.S. security point of view, but for keeping the balance of power on the mainland [of Asia], China is more important."

In July 1971, Kissinger made his dramatic secret visit to Peking -- a date some analysts believe was more significant for Asia than the April 30, 1975, fall of Saigon -- and paved the way for the historic Nixon trip of 1972.

The Sino-Soviet split was crucial to improved relations between China and the United States. But the U.S. failure in Vietnam also provided an impetus.

The U.S. policy of "containment" in Vietnam was aimed fundamentally at preventing the spread of communism from China, said Sutter. "If Vietnam had not been a failure, would we have had the political clout to change that [policy]?"

From the Chinese point of view, there is no clear evidence that the war postponed establishment of U.S.-China ties. There is considerable evidence, however, that the two countries had to go through a period of hostility -- with China trying to shake off any signs of foreign domination -- before firm ties could be established.

The official Chinese line has been that Peking's support of Hanoi was justified. Its cost apparently went beyond the $20 billion in weapons and supplies the Chinese say they supplied to North Vietnam. Earlier this month, the Chinese ambassador to Vietnam participated in a ceremony honoring Chinese who had given their lives in the Vietnam War, according to a Chinese news agency report from Hanoi. These Chinese "martyrs" apparently included railroad workers and troops keeping open the railway supply line from China to North Vietnam and possibly some Chinese military advisers working with Vietnamese antiaircraft crews and other units, according to the report.

But the Chinese are clearly in no mood to draw public attention to Vietnam's triumph 10 years ago over the United States. In private, some Chinese expressed regrets for China's past support for Vietnam.

As one Chinese intellectual put it, "We helped to nurture a wolf, and we've paid dearly for it."

From the 1960s until the early 1970s, the road from Friendship Pass in China's southern Guangxi Province conveyed Chinese materiel and aid to Hanoi. By early 1979, that same road conveyed the Chinese troops that invaded Vietnam.

The chain of events in Indochina has also acted to undermine insurgents in the region. In effect, the events since 1975 -- the huge exodus of Vietnamese refugees, the bloodbath in Cambodia, the people continually crossing the Mekong River from Laos, the abject poverty of the Indochinese states and the evident bankruptcy of their economic systems compared to the prosperity of the noncommunist Association of Southeast Asian Nations (ASEAN) -- have all been very poor advertising for communist revolutionaries in the region.

Also as a result of the Sino-Vietnamese conflict, China has had to team up with ASEAN and cut back its aid to Communist rebel movements, notably in Thailand, but also in Burma and Malaysia. The Philippines, with a basically indigenous, self-supporting Communist insurgency, is a case apart.

In Vietnam, there is a clear sense that the country is in danger of being left behind permanently by other, dynamic Asian economies. Vietnam's per-capita gross national product of about $160 is about one-fifth of Thailand's. Even in war-ravaged Cambodia, which suffered the worst agony of the Indochinese countries, people live better than in Vietnam these days, according to recent visitors. That may explain, in part, why so many Vietnamese are moving to settle there.

"Ten years after the fall of South Vietnam, I see no progress in reconstructing the country," said Prasong Soonsiri, secretary general of the Thai National Security Council. "They are hurting themselves. Instead of taking the ropes from their necks, they are bringing more rope to hang themselves."

Vietnam's most anxious glances are turned toward its huge northern neighbor. Not only does Vietnam view China with hostility and trepidation, but the glances contain a hint of jealousy over China's recent economic successes.

There is no question, however, that the overall Sino-Vietnamese hostility "is a plus for the psychological health of Southeast Asia," according to one Reagan administration analyst.

That, in turn, has benefited the economic health of the region. Since the American involvement in Vietnam, the other "dominoes" -- the noncommunist countries of the region, with Japan in the lead -- have increased their output of goods and services more rapidly than any area in the world. During the last 10 years, the ASEAN countries have been growing an average of 7 percent annually in real terms (after inflation), which is about twice the global average.

Japan, with its success in rebuilding after World War II, has become the noncommunist world's second largest economy. It is envied among its Asian neighbors, some of whom look to it as their model, and it is the most important trading partner for most of the Southeast Asian countries, providing a stable market for their raw materials and a steady flow of development capital.

By many accounts, Japan owes some of its high-velocity economic growth in the late 1960s to U.S. military spending in Japan. But it is harder to show a direct cause-and- effect relationship between the war and Japan's current economic strength and increasingly important security role in the Pacific.

Ten years after the war, the overwhelming American military presence in Vietnam does not seem to be missed much in the region. Although Philippine President Ferdinand E. Marcos is increasingly seen in Washington as a net minus for U.S. interests, there is no question of putting in a replacement or even of being seen to be casting around for one.

"We're not in that business anymore," said one U.S. official in the region. "Those days are long gone, and we were never very good at it."