SINCE THE fall of Saigon in 1975 ended its exportation to the United States, Washington's Vietnamese have craved their national soybean sauce. It's called Tuong, and it is quite different in taste from the Japanese or Chinese varieties. Although there are many brands of Oriental light and dark soy sauce on the American market, some produced in this country, the Vietnamese version is much lighter in color and thicker in consistency, with a slightly sour taste.

Now this soy sauce is being produced here, in Bowie, Md. Chemist Hoang Van Chi, his wife, Hoang Phan, and many relatives have started a small business called Vietnam Food and Drink in the Ardmore industrial park. Their first shipment went out in June -- 4,000 pounds to Paris, where the largest concentration of expatriot Vietnamese live. The second shipment went to California and the third to Texas. American soybeans from Kentucky and Maryland are used.

Preparation of all soy sauce follows a general procedure. First, the soybeans are roasted and cooked. Then they are fermented. This process breaks down the proteins into about 17 amino acids, giving the sauce its distinctive flavor. Meanwhile, rice or wheat flour is fermented by a mold to change the starch contained in the grain into glucose and maltose, which gives the sauce a sweet taste, its pleasant color, fragrance and low acidity. When the soybean and rice fermentation are complete, the two products are combined. Then salt, and occasionally sugar, is added.

After the fermentation process is completed, Chinese soy sauce is filtered, and all remaining solid particles are discarded. Tuong, however, is ground, producing a thicker substance. Where the Chinese add lactobacillus to the cooked soybeans to provoke the fermentation process, the Vietnamese roast the soybeans before cooking, which accounts for the different taste. The roasted beans also produce the brown color. Chinese soy sauce becomes black only after it is exposed to the air for about three months.

Some manufacturers add molasses or caramel to speed this process.

The final products do look and taste quite different. For a Vietnamese cook, Japanese soy sauce is a foreign product. Think of a chef from Dijon tasting America's sweet mustard. Vietnamese restaurants catering to American tastes, however, put soy sauce on the tables for their customers.

Another important cultural difference: Tuong is used as an accompanying condiment with such dishes as Vietnamese pancakes, deep-fried Tofu, or steak.

It is not used instead of salt as is soy sauce in Chinese and Japanese cuisine. For that these Southeast Asians have Nuoc Mam, made from the fermentation caused when fresh fish is layered in barrels, alternating with salt. Similar fish sauce is available in Thailand and the Philippines, and is easily obtained in American markets.

Word about the new domestic source of Tuong has spread rapidly in the local Vietnamese community. As a result, after the Hoangs' export orders are filled, they have more than they can do filling local orders.

The Hoangs are well known in the Vietnamese community. A chemist by education, Hoang Van Chi designed a hydroelectric power plant in Vietnam. Later, he wrote his now-classic "From Colonialism to Communism" before becoming a voluntary exile in 1959. In 1965 the Hoangs came to the area and Hoang worked for AID as a specialist in Oriental culture. After retiring from public duty, he returned to his original profession. With the soy sauce well on its way to success, he is experimenting, trying to find a formula to produce nourishing dishes from soybeans and rice.

This week the Hoangs are taking time away from their factory to demonstrate Vietnamese cooking at the Smithsonian Folklife Festival. From 10 a.m. to 5 p.m. daily (except on Sunday) they will be demonstrating. DAU PHU RAN CHAM TUONG (Fried tofu dipped in Vietnamese soybean sauce) Vegetable oil for deep frying Tofu or bean curd cut in large chunks

Pour the oil in a heavy frying pan to a depth of about 2 inches. Heat until it reaches 375 degrees, then add a few pieces of tofu. Fry on each side, remove to a paper towel to drain, and serve with Tuong. BO TAI TUONG GUNG (Steak with Soybean Sauce and Ginger) (4 to 6 servings) 2 pounds flank steak 2 teaspoons MSG (optional) 2 tablespoons nuoc mam (fish sauce) 1 teaspoon white or black pepper 1 medium onion, thinly sliced 2 tablespoons white vinegar 1/2 cup Chinese parsley, coarsely chopped 1/2 cup Tuong or imported soy sauce 1/2 inch grated ginger root

Trim off any fat from the flank steak. Cutting on the bias, slice lengthwise into three long pieces. Marinate the meat with the MSG, fish sauce and pepper for at least an hour.

Broil in a preheated oven for 3 minutes, turn over and cook another 3 minutes. The meat should be rare.

Remove the steak from the broiler, cut into thein slices and serve topped with a thin layer of onion which has been dipped in vinegar. Top with some Chinese parsley.

Use chopsticks to dip the meat, onion and parsley into the soy sauce to which the giner has been added. BANH XEO (Pork, Shrimp and Vegetable Pancake-Omelet) (6 servings) 1 cup rice flour 2 cups water 1/2 cup coconut milk 1 tablespoon sugar 1 teaspoon saffron (optional) 1/2 to 1 cup finely chopped green onion tops 1/2 pound pork belly, sliced thin and cut into 2-inch squares. 1 medium onion, sliced very thin 12 shrimp, thawed, pealed and vein removed Oil for frying 1 pound fresh bean sprouts 2 eggs 2 tablespoons water Leaf lettuce Fresh mint leaves Chinese parsley Thin slices cucumber Nuoc mam Tuong or imported soy sauce

Mix the rice flour, water and coconut milk to form a thin batter. Stir in the sugar to make the cooked pancake crunchy. Add some saffron, if available, to give the batter a touch of color. Finally, gently mix in the finely chopped green onion tops. After the first pancake is cooked, if the batter seems too thin, add 1 or 2 tablespoons plain flour. If too thick, add a little more water. It should, however remain very thin and runny.

Using a very sharp knife, slice each shrimp into 3 thin lengthwise slices.

Heat the oil to 375 degrees. To make the first pancake: Put in 3 or 4 slices of the pork and 3 or 4 slices of the onion. Saute on high heat, stirring constantly, about 1 minute. Add 2 or 3 pieces of shrimp and cook, stirring, for another few seconds.

Pour 1/2 cup of the batter over the pork, onion and shrimp. Drop a handful of bean sprouts over the top of the pancake.

Mix the eggs together thoroughly, then mix in the water. Dribble 2 tablespoons of this mixture around the outer edge of the pancake. After the pancake has cooked only a few seconds, cover the skillet and let it continue to cook on medium heat about one minute. Uncover the skillet and fold the pancake in half with a spatula.

Continue with remaining pancakes and batter. Serve hot. To eat, wrap a portion of the pancake in a lettuce leaf with mint, Chinese parsley and cucumber. Dip in Nuoc Mam or Tuong.