TO RETURN TO THE RICE PADDY and bamboo region of East Asia after five years' absence -- especially these five years, after the fall of Saigon brought a crisis in the American role -- is to experience both the familiar and the unexpected, both continuity and change.

Malaysia, Thailand, Vietnam, South Korea and Japan, which I revisited on a recent month-long journey, are the modern outcroppings of very old civilizations. Thus it is understandable that continuity is the stronger sensation. Half a decade, after all, is only a fleeting moment in the perspective of the Orient, even if this particular half decade of 1975-1980 has been a moment of shifting antagonisms and relatively great fluidity.

My trip began in the press party of Secretary of State Edmund S. Muskie. He flew late in June to join a meeting of southeast Asian foreign ministers in Kuala Lumpur, Malaysia, as a demonstration of continuing U.S. interest in and support of these Asian nations.

When Muskie and his retinue flew home across the Pacific, I remained behind to take a new look at several Asian countries I had known well in earlier years.

My notebooks and recollections at journey's end were a jumble of impressions -- a motorboat trip up the Chao Phrya River in Bangkok to the Temple of Dawn on a sultry day; a four-hour auto ride from Hanoi to the embattled Chinese-Vietnamese border, chauffeured by a handsome former sergeant who fought Americans at Khe Sanh and the Ashau Valley and who was wounded by U.S. shells in the head and hip; an elegant Korean dinner in Seoul, where I grimly chewed and hurriedly swallowed a wriggling piece of live squid wrapped in a lettuce leaf; the identical bamboo fans fingered in identical fashion by the new prime minister and foreign minister of Japan in their separate offices in Tokyo.

My strongest conclusions, as I sorted out my thoughts on returning home, were these:

Most obvious -- and often overlooked because it is obvious -- is vitality, enterprise and economic growth.

Traffic jams in Bangkok, lofty new buildings in Kuala Lumpur and Seoul, the continuing cross-the-board affluence of Tokyo -- they are all dramatic symbols of prosperity. Non-communist Asia in the past decade has been among the most rapidly growing areas of the world. The ordinary citizens, with few exceptions, have received a substantial share of the larger pie.

In the wake of the redoubling of world oil prices last year, most of the Asian countries have experienced the painful spurt in inflation and decline in growth which has afflicted the rest of the world. If this persists over the long run, it will be a grave threat to internal stability as well as prosperity, but so far the setbacks have been considered temporary and surmountable.

The glaring exception to the evidence of growth is Vietnam, which has not recovered from 30 years of revolutionary war and has not found a way to replace an externally subsidized market economy in the south with a functioning socialist system. Vietnam's economy is not working. It is in continuing decline, with no basis for a turnaround in sight.

Vietnam, despite the departure of American troops and the end of the U.S. war, remains at the center of regional conflict.

The warrior state of Southeast Asia scarcely paused to celebrate its victory before undertaking new military struggles along its China border and in Cambodia. Both conflicts have deep historical roots, but in Hanoi's view, the present battles in both cases were forced upon Vietnam since 1975. The Southeast Asian neighbors, on the other hand, tend to believe that Vietnam chose war over diplomacy because fighting is what it best knows how to do.

The Vietnamese speak of the struggle with China as another protracted war on the model of the earlier ones against France and the United States. The theory is that China, like the western enemies, eventually will tire of the fight and give up. This may well be a faulty analogy. The United States may recover from its "Vietnam syndrome," -- the political and psychological effects of defeat -- before the Vietnamese recover from the political and psychological consequences of their victory. China, unlike France or the United States, will never go away.

The Cambodian battle threatens to spread westward to Thailand and if so, to bring the rest of Southeast Asia, the Western world and China into even greater collision with Vietnam. It is unlikely that Hanoi wishes to spread the battle to Thailand any more than Washington wished to spread its war west to Cambodia, but similar imperatives apply. The Vietnamese find Thai sanctuaries for their enemies intolerable, just as the U.S. found the Cambodian sanctuaries intolerable a decade ago. The ironies and parallels abound.

One very clear impression, based on contact with Vietnamese officialdom and the foreign diplomatic corps in Hanoi, is that it makes no sense for the United States to be unrepresented there. America's European and Asian allies, including Britain, France, Japan and Thailand, are represented by respected ambassadors; as are Vietnam's Communist friend and foe, the Soviet Union and People's Republic of China, respectively. All the players in the Vietnam drama are present except one -- the United States. The Vietnamese have set aside a large villa at 19 Hai Ba Trung as the future site of a U.S. embassy after normalization of diplomatic relations. It was the U.S. consulate in Hanoi during the French era.

In Northeast Asia, Korea remains the potential flashpoint of conflict, in a situation made far more unstable by the struggle for political succession in Seoul.

Internal maneuverings in the nine months since the assassination of President Park Chung Hee last Oct. 26 have brought forth a stern military rule, thinly masked by a facade of civilian administration. The new leader, generally known in the western press as "the strongman," is Lt. Gen. Chon Doo Hwan. In uniform garb the chief of the military intelligence agency, in civilian clothes he is the head of the standing committee of a super-government, the shadowy "special committee for national security measures."

Chon, who is a little more than 20 years younger than the late president, was his loyal supporter and protege. Chon is from Park's home province and was a junior aide to Park immediately after the 1961 coup which brought Park to power. Thereafter, the late president sponsored Chon in a variety of sensitive military posts, including key positions in the presidential security force.

In person, Chon is a chunky, balding man of 47 who gives the impression of tautness, mental and physical. I found him more self-assured and politically sophisticated in his responses to questions than was Park, whom I interviewed in 1975. At the same time, Chon faces far greater problems of public acceptance than did Park in his early days in power.

Talks with Chon and Lt. Gen. Ro Tae Woo, a close associate who heads the important Seoul Garrison Command, left me wondering whether the new leaders will be flexible enough to obtain and maintain the consent of a historically fractious Korean populace. The generals seem single-minded in their demand for order and security. But even the authoritarian Park learned it was unwise to keep the lid on too tight, too long, lest explosive pressures against his regime build up at home and abroad.

Chon and his fellow generals pay lip service to the U.S. relationship, which is essential for security against the communist north, but "they do take us for granted," said a senior American official in Seoul. There was no advance inkling of the Dec. 12 coup-like movement, which violated ground rules of American control of Korean front line units. There was only 15 minutes notice on April 14 that Chon was to be named acting director of KCIA, in the face of American misgivings. There was no advance notice to the U.S. that the generals were taking direct control of the country under total martial law May 17.

Some 40,000 American troops are still on guard against North Korea, 27 years after the armistice. President Carter's pullout of ground troops is at least temporarily stymied. Nevertheless, the costly and hazardous U.S. military commitment is likely to be questioned again in the 1980s, especially if the South Korean generals add internal instability to external danger.

Japan, despite economic and political stress, has opted for continuity and stability in the forseeable future.

Like Sherlock Holmes' clue of the dog which did not bark, the anticipated political change which did not occur this summer may be highly important. Following the sudden death of Prime Minister Masayoshi Ohira, the electorate gave a surprisingly large vote of confidence to the conservative party which has ruled Japan with only minor exceptions since World War II. A vote of dissatisfaction had been expected to force the conservatives into a coalition for the first time, marking the beginning of a process of extensive political change.

Japan is by far the most economically powerful country in Asia, third in the world in national production after the United States and the Soviet Union. Average per person income has crept ahead of many European countries and the U.S.S.R., and is gaining swiftly on the United States.

In the absence of sudden reverses or serious scandals, a relatively unknown insiders' politician named Zenko Suzuki is guaranteed a tenure as prime minister of at least two years. In his first interview since taking office a little over a week before, Suzuki struck me as a solid, competent, unexciting leader for a country where bureaucracy and consensus reign.

The Suzuki era seems to spell a little more of the same on every front: a little more economic management and a few more taxes at home; a little more foreign aid and military spending and a little more political responsibility abroad. The gradualistic approach may not please anyone very much, but neither will anyone be very unhappy.

East Asia's military environment has been changing more rapidly than its political affiliations and preferences.

Soviet military muscle has been increasingly visible, whether based in Siberia or the Soviet-occupied Japanese islands in Northeast Asia, or operating from the Vietnamese ally's Camranh Bay and Danang bases in Southeast Asia. China, in response, has stepped up efforts to support Cambodian insurgents and to "bleed" Vietnam along the Sino-Vietnamese border.

Unrestrained political warfare and even shooting wars through allies and proxies between the two giants of communism, Russia and China, are among the most important international developments of the late 1970s. East Asia has been the principal combat zone.

As a great power actor, the United States no longer dominates the East Asian stage as it once did, but this is all the more reason, in the Asian view, for Washington to play a strong role. The U.S. remains of great importance economically, politically and militarily in Japan and Korea, and of substantially greater importance in Southeast Asia than was projected at the fall of Saigon five years ago. The concern expressed to me in non-communist Asia was whether the U.S. leadership is and will be strong and purposeful enough to suit Asian needs.

Three times in the past four decades the United States has expended immense amounts of blood and treasure in East Asia -- in the World War II battle against Japan, in the Korean war, in the Vietnam war. While a new U.S. war in Asia seems unlikely at present, it must not be forgotten that the U.S. remains militarily committed to Japan, South Korea and Thailand.

Whatever the difficulties of East Asia, they appear modest by comparison with the revolutionary conditions in much of the rest of the globe. Given its vitality and resilience and its historic ability to adjust, the rice paddy and bamboo region seems likely to survive the storms of the 1980s as well as any on earth.