Nguyen Van Thu, 57, watched nervously as a customs officer dumped the contents of a package from his son in Canada onto a table and began sorting through western medicines, clothing and other merchandise.

A woman, Tang Ly, fidgeted nearby while another official read a letter included among the boxes of aspirin, Bic pens, T-shirts, underwear and material a relative had sent her.

"We live off these parcels more than anything else," said a 25-year-old government employe. "Without them we couldn't survive."

Such scenes are replayed every day at a warehouse-like building on Co Giang Street in Ho Chi Minh City, formerly Saigon. Called Fidimex, for First District Import-Export, the place distributes tons of packages sent home by Vietnamese who have fled abroad, many of them illegally by boat.

Government authorities know that most of the goods will wind up on the city's thriving black market, bringing prices many times higher than the original. But they turn a blind eye, knowing also that the irreplaceable influx provides a great boost to the economy.

Although Communist Party ideologues frown on it, the traffic gives Vietnam a vast array of western products without costing the country its scarce foreign exchange. The goods, estimated to cost more than $70 million a year in the West, also fill Vietnamese coffers with millions of dollars in customs revenues and give western transport companies at least $26 million a year in business.

According to Ngo Van Phuong, director of the Fidimex parcel distribution center, authorities distribute 800 to 1,000 packages a day with declared values averaging $200 each.

With duties ranging from 15 percent for medicines to 200 percent for cosmetics, he said, customs officials take in 400,000 to 800,000 dong a day ($41,000 to $82,000 at the official exchange rate).

Nevertheless, Phuong conceded, "The government does not encourage people to receive parcels."

Last October, the Vietnamese Army's theoretical journal sharply attacked the system of "family packages," which it said were "sapping our economy."

"Most of these parcels contain luxury items made in Japan, America or other western countries," the journal said. "Once these foreign items come in, they revive the neocolonialist American market, disrupting the development of national manufacturing, the normal exchange market and prices."

Such denunciations have had no effect, however, on the influx of goods or the government's apparent desire to facilitate it.

Despite numerous steps and a thorough inspection, the distribution seems to work fairly smoothly.

Phuong asserted that while there have been "lots of losses" at the city's airport because of "bad employes," almost all of the parcels that arrive at the distribution center get delivered to their intended recipients. Out of 965 tons the center has handled since April 1981, he said, only 140 pounds have been lost, and that was to thieves using fake papers.

The only products confiscated are those considered unhealthy or otherwise dangerous. Among the banned items are cigarettes and tobacco, fresh foods, certain kinds of Chinese medicines and Tylenol, whether made in the United States or not.

During a recent visit, one of the few items that authorities confiscated was a jar of San Ing facial cream, made in Taiwan. "They say it's dangerous for the skin," one official said. "I don't know why."

The goods that go through evidently provide a major bonus to families in a country where salaries are abysmal. A government employe who receives a parcel a month from brothers said she earns 200 dong a month ($20.61 at the official rate) at her job, but can make 5,000 to 6,000 dong ($515 to $618) a month by selling her package on the black market.

So elaborate is the system that many packages reportedly are sold in advance to black market buyers to raise money for the customs duties.

According to Phuong, the largest share of the packages comes from France. The United States, with a Vietnamese population now estimated at 420,000, also provides a major portion. According to one estimate in California, 20 to 25 percent of Vietnamese residents' disposable income--income not directly channeled into food, rent and the like--goes for these "care packages."

The influx also apparently contributes to the parallel black market economy that has made the official exchange rate irrelevant in some sectors. In the cafes and nightclubs that have sprouted up here under government or joint state and private ownership, drinks can be bought only at black market prices, as in the days before the Communist takeover of South Vietnam.

At one nightclub owned by Fidimex, for example, a Coca-Cola costs 150 dong, which would be $15.46 at the official rate but $1.42 at the black market rate. A Heineken beer costs the same, but, for some reason, a dry martini costs only a third as much.