After what an onlooker called "the first really good fight they've had in a long time," China's 4-million-member army faces at least a decade of work in applying the lessons learned in a month of battle on the hills of Vietnam.

Experts here have been watching the two-year-old campaign to modernize China's People's Liberation Army say it will take at least that long to acquire the needed weapons and combat techniques. Peking's goal is to be able to confront China's major enemy, the Soviet Union, with as much confidence as they showed in pushing back the Vietnamese.

According to some speculation here, China's generals now may be able to win budget increases from a pennypinching Communist Party leadership because of increased fear of a Soviet threat in the wake of the war with Vietnam, and because of military weaknesses revealed in the border fighting. There is no evidence of increased Chinese military spending yet, however, and some analysts argue the border war may have diminished the Soviet threat.

For the next 10 to 20 years, however, while the Cninese buy new weapons from the West and train their soldiers to use them, Peking's generals are expected to be a watchful and worrisome lot. Chinese officials have told foreign vistors orivately that they fear a Soviet attack while China's forces are still in this building stage.

To the minds of many Chinese here and in Peking and many foreign analysts, the invasion of Vietnam was simply a warning to Moscow not to take advantage of this time of apparent weakness. A foreigner who saw Vice Premier Deng Xiaoping (Teng Hsiao-ping) during the border war described the Chinese attitude. They cannot tolerate any aggression against them that could be construed as indifference or weakness."

Western military analysts admit they do not know what route an invading Soviet army, intent on destroying China's growing military might, would take. One said that if he were commanding the Soviet forces, he would simply launch a fleet of bumbers from near the far eastern Soviet city of Vladivostok, only 50 miles from the Chinese border, and bomb the industrial cities of Manchuria.

To be able to counter such an attack, the Chinese must build or buy a new fleet of antiaircraft missiles to replace the old 20-mile range SA2s now in their arsenal. They have shown interest in Britain's Rapier and France's Crotale, among others.

To bring down the Soviet Backfire bomver or control the air against Soviet fighters, the Chinese must revamp their large but aging air force of Mig 17s and 19s, which has only 80 Mig 21s and few Chinese-made F9s. The Chinese have expressed interest in buying the Mirage 2000 interceptor from France, and there have been conflicting reports on whether the French will ignore Soviet displeasure and make such a sale.

The Mirage is only one of a number of European-made weapon systems in which the Chinese have expressed interest yet signed no firm contracts to buy. Peking has moved as slowly in this as in other trade areas, window shopping everywhere and signing only the most preliminary protocols even after apparently making a decision. The Chinese Military budget does not appear to have grown significantly, if at all. By one estimate (the Chinese budget is secret), Peking now spends about 8.5 percent of its gross national product on the military,mpared with 10 percent in 1976. The budgeted amount is estimated at about $35 billion. All the official Chinese press discussion of military spending in the last several months has indicated a decision to build the civilian economy first.

To prepare for an invasion of a mobile, tank-rich Soviet land army, China must modernize a fleet of 10,000 tanks, mostly old Soviet type T34 and T54 models. The Chinese have shown intense interest in antitank weapons, giving a close look to the Frensh "HOT" and "Milan" systems.Military analysts here suggest that the Chinese effort to buy the British Harrier jump-jet, another deal vigorously opposed by Moscow, is part of this search for effective and modern defenses against Soviet tanks.

In this area, analysts think the Chinese may have had some interesting lessons in Vietnam. The Vietnamese did commit tanks to the crucial battle for the town of Lang Son. Westerners who visited the frout said they heard the distinctive sound of the Soviet Sagger wire-guided antitank missile, fired by the Vietnamese at the Chinese armored vehicles. If these reports are accurte, it would have provided the Chinese a useful taste of what they could expect in a clash with the Soviets.

Although the evidence is inconclusive so far, some analysts detect in the fitful beginnings of the Chinese invasion into Vietnam a poorly coordinated division organization that lacks the tight integration of tank and motorized rifle divisions used by the Soviets. The Chinese appear to have been working for the last two years to modernize their organization and train a new generation of officers to command it.

The Chinese over the last several months also have shown interest in American weapons, forcing a Carter administration decision to rule out such sales. Analysts here say they have been surprised at the volume and expenses of the Western weapons systems that Chinese military delegations have actively priced, but wonder if the strain of putting the Chinese civilian economy back on its feet might restrict the pace of their actual military spending.

If the strike into Vietnam increases the immediate threat to China's northern and southern border, Peking may spend more. If the Vietnamese and Soviets now back off, as the Chinese hope they will, the big purchases may not come nearly as fast as Western arms manufacturers would like.