Vietnam is suffering a critical food shortage that already is causing undernourishment, and some authorities expect it to endure for years unless there is a massive infusion of foreign aid.

The shortage has become so severe, according to both government and independent sources, that many workers are not eating enough to sustain them satisfactorily for a full day's work.

State rations to many Vietnamese have been reduced twice in recent months, and the cost of food on thriving private markets has soared to levels five to ten times that of government-controlled prices. A single chicken at one private market here bears a price tag of about $12. That equals about half of the average worker's monthly wage.

The conditions are not as bad as in neighboring Cambodia where famiine is likely this fall.

"Cambodia is disastrous," said one food expert. "Vietnam is critical, but they are not approaching famine here."

He said, however, that because of a steady deterioration in Vietnam's crops over a long period the shortage may last for several more years;

"They cannot get out of the (food) deficit for the next five years without help," he added.

The devastation of wars and a series of natural calamities -- drought, floods and insect pests -- are the major reasons that Vietnam, which has basic resources to feed itself, is suffering.

In addition, Vietnamese authorities claim that part of their rice crop is being sent to Cambodia, which has a Vietnamese-backed government and Vietnamese troops stationed there. They decline to estimate how much is being sent to Cambodia. Independent sources are able to confirm only that rice seedlings for planting are being exported to Cambodia.

The dimensions of the food shortage are larger than previously anticipated, partly because of devastation resulting from the Chinese invasion.

Mai Luong, vice director of the Agriculture Ministry's international cooperation department, said in an interview Saturday that Vietnam this year needs to produce 19.3 million tons of food crops, mostly rice. He estimated only 15.5 million tons will come to market, leaving a shortfall of 3.8 million tons.

Part of that deficit is to be made up by contributions from socialist countries, principally the Soviet Union, and by purchases from other Asian nations, he said. Luong said he expected about 1.5 million tons to come from foreign gifts or purchases, and other sources made approximately the same estimates.

Luon is not optimistic about the near future. He said the autumn rice crop will be poor because heavy rains submerged seedlings as they were being planted.

Government officials talk freely to foreigners about the shortage, asserting that the major causes -- man-made and natural -- are beyond the government's control. Luong cites the following reasons:

The war with the United States left vast destruction, including damage to 1,600 irrigation systems by American bombs, the killing of 40,000 water buffalo, the wreckage of experimental farms, and the devastation of thousands of acres of forests and rice paddies, some of which are still out of cultivation because of land mines left behind.

The invading Chinese destroyed more than 100 state farms, forests, and nursery gardens. China's troops either killed or made off with 250,000 pigs and 160,000 buffalo and cows, burned thousands of acres of forest and left a large number of riceland plots along the border too ravaged for planting this spring.

A series of natural disasters have afflicted the country since 1976. These include a bad drought in 1977, heavy floods in 1978 and a subsequent infestation by insects that destroyed crops.

Independent sources said these explanations are generally accurate, although they could not confirm the government-compiled statistics. They said there had been a noticeable decline in living standards since China cut off food aid last summer, and they agreed, from personal observations, that many workers are so undernourished that they are too weak for a full day's work.

"They get enough to live on, but not enough to be comfortable on," said one diplomat.

Another reason for the food shortage, ironically, is the refugee exodus, which the government once deliberately hastened. The refugees have taken so many fishing boats in their flight that fishing production is down.

Nguyen Co Thach, the secretary of state for foreign affairs, told reporters last Friday that the refugees had "stolen" more than 5,000 boats, cutting fish supplies by half. Other sources confirmed this in general without vouching for the numbers.

Vietnam is attempting to make up part of the fish shortage by promoting a national campaign of aquaculture in what are known as "Ho Chi Minh ponds."

There are reports here that another major reason for the food problem is the refusal of the rebellious private farmers in southern Vietnam to grow any rice for markets, seeding only enough each crop period to feed their own families.

The reports could not be confirmed here. However, Luong acknowledged that his ministry has benn unable to establish many new cooperatives in the south yet.

"Our intent is to eliminate the system of exploitive landlords there," he said. "We are doing this, but it is not yet done."

Thach, the government's principal spokesman to the foreign media, indicated that one reason for low production in the south was the American withdrawal in 1975. Farmers there no longer can get fertilizer and pesticides, and they now lack spare parts for farm machinery imported from the United States, he said.