Asia's most battle-hardened fighters, the Vietnamese, have entered the 1980s locked in a new, protracted war against an ancient enemy, the neighboring hordes of China. Despite the great disparity in size, Vietnam is officially confident of eventual victory.

"We have known them for 4,000 years and have repelled them many times," said Foreign Minister Nguyen Co Thach, sitting in a airy reception room in the old French-built ministry. "China dominated us once for 1,000 years, and a second time for 30 years. In one century there were three Chinese invasions. Our longest period of peace with them was 300 years."

Because the conflict with China seems to be destiny as well as history, Thach and other officials said, Vietnam is neither surprised nor intimidated by the turn of events since the end of the war against the United States five years ago.

The conflict in Cambodia, where Vietnamese troops are fighting Khmer guerrillas at or across the tense border with Thailand, is spoken of here as secondary to the main struggle with the Chinese. Little is said of Vietnamese ambitions for a unified and Hanoi-led Indochina. Pol Pot and his Cambodian guerrillas are seen as deriving their strength from China, pawns in a bigger war. As in the American-Vietnamese conflict of the past, Cambodia in this perspective is again a "sideshow" to the main event.

Other parallels with the past arise frequently. The evidence is strong that Vietnam's seemingly unbounded confidence against the larger enemy stems more from the victory over the French in the 1950s and over the Americans in the 1970s than from the mixed historical record of conflict with China.

It is far from clear, however, that China is subject to the same vulnerabilities as was the West. Public opinion, for example, seems relatively unimportant in Peking. While Thach and some other Vietnamese officials say China will tire of protracted war just as the French and Americans did, other Vietnamese concede that political repression in China makes a change of heart more difficult.

Moreover, unlike France and the United States, China is fixed force on Vietnam's border. China will never "go home" as the Western armies did. Some Vietnamese intellectuals worry that their faraway Soviet ally is more likely to be irresolute than the neighboring Chinese enemy.

At the moment, relations with the Soviet Union seem close, particularly following the visit to Premier Pham Van Dong, Communist Party Chairman Le Duan and other leaders to Moscow a few days ago. Soviet advisers and technicians can be seen in the streets in Hanoi, Saigon and other cities, and Vietnam is heavily dependent on Moscow for its imported petroleum and extra rice needed to feed its people as well as for military supplies and spare parts.

There is speculation among Hanoi-based diplomats that Vietnam called the recent top-level meeting. There is no indication of why Vietnam felt such a session necessary at this time, or whether any important changes will result. As long as Hanoi-Moscow relations remain close, of course, Hanoi-Peking relations will be severely strained.

In total size and weight, Vietnam is far outclassed by its giant neighbor. China has armed force of 4 million and a population of 1 billion compared with Vietnam's 1 million-member Army and 54 million people. China, however, also has a vast continent to police and defend, and a superpower enemy threatening its own northern border.

"Our troops are better seasoned than the Chinese after 30 years of war against the French and Americans, and we have better equipment," said Col. Tran Quang Man, editor of Vietnam's Army newspaper and a leading military commentator. Man quoted a Vietnamese proverb that says, "One Vietnamese soldier can defeat 200 Chinese soldiers."

Gen. Van Tien Dung, architect of the 1975 assault on Saigon and now Vietnamese defense minister, boasted that "the outstanding success of our military science and art . . . lies in the fact that we have successfully solved the problems of using a small force to defeat a big force."

Col. Man, explaining this asserted feat, said the Vietnamese secret is to combine the strength of the Army and people and to fight in the political and psychological fields as well as the military arena.

Despite the comforting doctrine, in practice the Vietnamese appear to be taking no chances. A trip by car from Hanoi 90 miles north to Lang Son, the final objective of Chinese troops in their month-long early 1979 border penetration, showed numerous fortified positions along the road. They did not appear to be currently manned, however.

Officials in Lang Son -- where public offices, schools, hospitals, the main marketplace and stadium were destroyed by Chinese dynamite charges during a brief occupation -- refused permission to travel the remaining 12 miles by road to the Chinese border. The ostensible reason was that the area is insecure, but the more likely reason is that its bristling fortifications are military sensitive.

Western intelligence reports, which could not be confirmed here, have said that Vietnam has about 250,000 of its best troops, armed with its best aircraft, tanks, surface-to-air missiles and other equipment, deployed against the Chinese along the northern border.

By comparison, the Vietnamese are reported to have deployed about 200,000 troops, including many raw recruits from the south, in the Cambodian battle. Vietnamese losses were reported by western sources to be 200 to 300 killed a week as fighting intensified along the Thai border late in June.

Apparently in reaction to growing Vietnamese threat to the Khmer guerrillas and the growing Vietnamese pressure on Thailand, China has increased its military pressure along the Sino-Vietnamese border. This appears to be a classical Chinese maneuver to "stretch thin" the Vietnamese, a tactic often used by the Vietnamese against the French and Americans in years gone by.

Officials at Lang Son, near the Chinese border, said the first major Chinese show of force in that area was in the form of intimidating maneuvers just across the border beginning Dec. 20, 1978. This was less than a week before Vietnam's Christmas 1978 invasion of Cambodia, and seems likely to have been intended as a warning to Hanoi, based on Chinese intelligence of what was to come.

The patient application of indirect pressures and sudden blows are more in the Asian tradition then the frontal, conventional and impatient strategies of Western armies, and they may be more effective in this setting.

"The Viets found the French easy, the Americans a bit tougher, but in the Chinese they have an enemy worthy of their mettle," a seasoned Western observer of the Hanoi scene said.

"One of the Vietnamese problems," according to this analyst, "is that they and Chinese are so much alike in thought and action. They are both playing by the same total absence of rules."