LIFE imitated art once again as Calvin Trillin, author of "Third Helpings," reached across the Formica table at the Mekong restaurant in Arlington for his third helping of Com Tam Bi.

That's "Broken Rice with Shredded Pork Skin, Beef," the sixth Vietnamese delicacy of the hour being consumed in Washington's "Little Vietnam" by the man who is trying to get the national Thanksgiving dish changed from turkey to spaghetti carbonara, the one who made the Buffalo chicken wing fly. Trillin, whose earlier books include "American Fried" and "Alice, Let's Eat," and who writes some of the most droll prose to ever whet the appetites of readers of The Nation and The New Yorker, was on a roll.

History is full of those who write of the little man. Trillin writes of the little food. His stomach is a sort of Library of Congress of hometown cooking, having catalogued and filed every Italian sausage ever served at the Feast of San Gennaro, not to mention most Louisiana boudins. And his exhaustive surveys of the gastronomic possibilities of food festivals and storefront restaurants and back-alley carryouts and taxi-drivers' snackeries from coast to coast and continent to continent are legend.

So when he came to Washington to do the talk-show and book-signing circuit, any thinking gastronome would have known what was called for: an eating tour of Clarendon.

"You guys are really lucky here," Trillin marveled as he passed Highland Street on Wilson Boulevard, where the jewelers and department stores are identified first in Vietnamese, then in English. The catch in his throat was pure envy as he walked through the doorway of My An, past the yard goods to the pool hall, filled at 4 p.m. with intent players sipping Southest Asian coffee soda.

"There is nobody who is more in favor of relaxed immigration policies than I am," he said. "The only people I wouldn't let in are the English." Immigration laws have been traditionally based on bland food, he explained, which is why the Northern Europeans and the English had such an easy time of it. And America's food has suffered for it. Trillin would rectify the situation; while he was watching the fall of Saigon on television, "I kept saying, 'Get the chefs! Take the chefs!' "

His plan hasn't been as effective as he would like; New York still has very few Vietnamese restaurants. As he put it, "It's not at the point where people say, 'Let's go get some Vietnamese food.' "

Trillin's appetite hadn't yet quickened as he was marched through the Pacific department store, up the back stairway past the travel office and watch repair to the store's cafe. He'd been "performing" at the Kitchen Bazaar, where most authors give cooking demonstrations, but Trillin had given an eating demonstration. Vietnamese music and a wall full of untranslated signs in the cafe brought hope. And the flimsy paper menu held promise.

How does he order in a strange Vietnamese restaurant? The same way he orders in a strange Chinese restaurant: Hand the waiter a note in Vietnamese that says something like, "Please give me some of what the man at the next table is eating."

What he got this day was roast bacon on rice and papaya shrimp. Since he had already eaten, and had several more "demonstrations" to go, he ate only two helpings of each. Trillin apparently will eat anything. Amend that. There is one food that makes him squeamish: Surfunturf. "I wouldn't eat one of those."

It was a grueling book tour. He had come from eating lutefisk in Minneapolis ("Everybody in Minneapolis hates lutefisk") and Italian beef sandwiches in Chicago (the beef having been marinated for "less than a year, I suppose") along with double-fried french fries ("the idea is to load them up one more time"), and was on his way to Pat's in Philadelphia for a cheese steak.

Trillin isn't really concerned with "decor," but he considers it important data in playing the percentages. His idea of a barbecue place with real possibilities is one where you put your plate on top of the jukebox to eat, under a sign that says "No Hair Combing in the kitchen." In fact, if someone suggests where there is great barbecue, Trillin immediately investigates: "Do they have plates?"

His usual method of finding a restaurant in an unknown town fails in Washington: "I always figure that you stick with regional specialties and the cooking of ethnic groups large enough to have at least two aldermen on the city council." But Washington has no region, and its food is what you might expect "from a town that's dominated by people who are willing to eat the sort of things that you serve at political dinners--for years--simply for a seat in the Senate." What you get is eating places where "the main creative effort of the restaurant goes to recognizing the people who come in."

Which doesn't mean he hasn't found worthy food, say, in Crisfield's and Germaine's and Sholl's ("I like to sit there and try to guess the GS rating of everybody in the place"). What makes a good restaurant town is not just having the people to open good restaurants but having the people to eat in the restaurants--a critical clientele. That is why a marvelous Italian restaurant doesn't work where there are no Italians.

By this time Trillin was again wondering where his next meal was coming from. The signs were good at the Mekong Center, where Moet et Chandon champagne nestled on shelves next to Vietnamese paperbacks. The music was different but every bit as Vietnamese, and the noise coming from the balcony restaurant competed with it. "This is the new hot place," he said over the din, his salivary glands lubricating his voice. The counter was piled high with dirty dishes, crushed beer cans and a serving bowl full of peeled garlic. Most tables were filled with Vietnamese wielding chopsticks, and five cooks bustled behind the counter, their chopping punctuated by the bell of a microwave oven and raucous laughter generated by Vietnamese jokes.

That was where Trillin discovered the broken rice and pork skin concoction, along with Rice Pancake with Special Patty Melt--which was sold out--and Viet Nam Rice Noodle with Beef and Pork Leg-Chili--which the waitress wasn't going to let him order without a fight.

"No good," she insisted.

Why is it no good?

"Very hot."

We want it hot.

It took two waitresses and a managerial-type guy in a tie and jacket to convince Trillin that he didn't want it, and still he wasn't convinced.

He got Viet Nam Rice Noodle with Beef and Pork Leg-Chili, but only after he also promised to try Special Beef Noodle Soup.

What does he ordinarily do when restaurateurs try to persuade him not to order the dishes that sound like the good stuff? "If it's ram's intestines I might be persuaded," he admitted, "but they don't want us to get it because it's hot, and they'll probably tone it down anyway. So what if you've ordered and it's too hot. You've lost $3. It's not like buying the wrong kind of car."

True, the pork-chili dish wasn't all that hot; it merited a mere hoarse cough and, "It does stick in the throat a bit." Also true, the beef noodle soup--Pho Dac Biet--was a sensation. "Oh boy, this is delicious. I told you this was the place," Trillin said, practicing his remarkable trick of talking nonstop while eating nonstop.

A happy man descended the stairs from the Mekong restaurant. "I'm glad the Reds didn't get these guys," Trillin sighed.

A quick peek in the Dalat down the block showed another mimeographed menu full of Phos and such, this one encased in plastic and pristine. And the counter in front displayed bright green translucent pillows that were some untranslated but alarming-looking food.

Not enough time to eat there.

Trillin's next demonstration was coming up, so the tour broke into a near-gallop. A quick duck into Fu Lo Bakery. Just everyday coconut tarts and assorted cookies. But on a side counter, where you'd only look if you knew they were there, was a pile of fried things studded with bits of black. "Fried fries," Trillin speculated. Sesame strips, the cashier revealed. Wonderful, Trillin discovered.

The Washington metropolitan area, Trillin was learning, had some pretty intriguing possibilities. But official Washington was bound to be another matter. The White House? "I think Trish Nixon should move right back in there with that chicken divan recipe. That was a fantastic recipe--pouring a lot of cans of chicken soup on a bird that was already a chicken," he muttered, but admitted he was bitterly jealous about Nixon getting to eat all that good food in China. "Why him?" he kept asking himself. Why the man for whom "growing in the presidency" meant learning to eat your cottage cheese without ketchup, he asks himself again and again.

Trillin has assumed that all the White House serves is chateaubriand or roast prime rib of beef au jus, the very enunciation of which dries up his salivary glands. As for Nancy Reagan, although "she last ate something in 1947, and that was a piece of melba toast," he observed, "She eats something that makes you smile all the time and never laugh."

Trillin is not merely a complainer, though. He is willing to come up with solutions for those hideous gastronomic problems that face our nation. Take the State Department Cafeteria, which any Washingtonian knows to be a very serious situation for our career diplomats. "I think that it would be a nice lesson to have it catered by various countries we've lost," proposed Trillin. "First they could have the Vietnamese for a week, and then the Chinese cater for a week. Everybody but the Albanians."

And if that doesn't work, he always has his Spaghetti Carbonara movement to fall back on. Here is the official recipe until there is a national convention set up to decide on a permanent version. SPAGHETTI CARBONARA (6 servings) 1/4 pound pancetta or lean bacon 2 medium onions, finely chopped 3 tablespoons olive oil Salt and hot pepper to taste 5 tablespoons chopped parsley, Italian if possible 1/2 cup prosciutto, finely diced 1/2 cup diced fontina or fontinella cheese 1 pound spaghetti 4 beaten eggs Grated parmesan cheese

Cut the pancetta or bacon into 1-inch pieces. Cook in a small skillet until crisp. Drain on paper towels and set aside.

Saute' onions in olive oil. When wilted, add all other ingredients except spaghetti, eggs and parmesan cheese. (If fontinella is used, add to sauce the last few minutes.) Cover and simmer over low heat, stirring often for 5 to 10 minutes.

Cook spaghetti in boiling salted water until tender, but fine to the bite, al dente. Drain, place in serving bowl, add eggs and toss well. Add sauce and toss again. Serve immediately with grated parmesan cheese. "Italian Family Cooking," by Edward Giobbi