The large but light-footed Russian glides gracefully across the floor to the strains of a tango, followed expertly by a lithe Vietnamese girl looking like a wisp against his enormous girth.

It is Saturday night at the Rex, and he is easily the snazziest dancer on the floor: the king, as it were, of the disco. %tFormerly a U.S. officers' quarters, the Rex is now officially called the Hotel Ben Thanh, but the government cadres, East European advisers and other foreigners who patronize its night club still know it by its old name.

For 20 dong an hour ($2.20 at the new official exchange rate but only about 50 cents at the black market rate), the patrons can rent a Vietnamese taxi dancer for an hour and hoof to the beat of a versatile Vietnamese band that plays everything from waltzes to disco.

In fact, some of the music is similar to the sort of Western tunes banned racently in this former South Vietnamese capital's cafes in a crackdown against "decadent" foreign influence. The main difference is that the Rex is owned and run by the government.

So now, on a Saturday night in Ho Chi Minh City, it is the only game in town. And at 11 p.m., in accord with the club's prudish regulations, the music stops and the Vietnamese dancers all shake hands with their partners and go home.

FOR AN AMERICAN first-time visitor, the city formerly called Saigon holds a lot of ghosts.

From the air, on the approach to Ho Chi Minh City, verdant paddy fields suddenly give way to a series of bomb craters now filled with rainwater. fAt Tan Son Nhut Airport, the hulks of U.S. military transport planes still line the runway. Off in a nearby field sits a jumble of more U.S. plans and helicopters, testimony to the haste and disarray of the American evacuation in April 1975.

In the city, a familiar building catches the eye, recognizable from news photographs on that day six years age when helicopters landed on its roof. Today the former U.S. Embassy houses Vietnam's state oil company, with a section reserved for the secret police. The U.S. ambassador's residence, meanwhile, has been turned over to the Soviet Union and now serves as its consulate.

In the streets of Ho Chi Minh City, the bars and clubs that catered to GIs have long been closed. So have many shops and businesses whose owners are among the hundreds of thousands of Vietnamese who have fled their country since the communist takeover in the South.

There are few cars on the streets these days, and the ranks of the old red-painted Chevrolets and Dodges that still serve as taxis are idle for lack of passengers who can afford them. Most people get around the city in the ubiquitous pedicabs, hal bide, half cart, called chclopousses , and many, including Vietnamese women with their distinctive conical hats and long black gloves, ride bicycles.

In the midst of all this, some reminders of the old days in Saigon can come unexpectedly, such as when one small boy, upon seeing a foreign visitor, runs up and calls out, "Hey, Joe."

OTHER WORDS and phrases are used when the foreigner is thought to be a Russian. This seems to be most of the time, to the fury of some East Europeans.

According to Westernresidents here, there has been tension between the Vietnamese and their Soviet advisers, especially in the South. The Soviets consider the Vietnamese lazy, these sources say, and the Vietnamese regard the Soviets as humorless drones who make them work hard with none of the extras they can expect from Westerners.

"The Vietnamese call the Russians 'stonefaces,'" says one Hanoi-based European.

According to another foreigner, "The Soviets haven't made an effort to understand the Vietnamese psychology as much as the French and to a lesser degree, the Americans did."

However, the impact of the estimated 4,000 to 6,000 Soviet personnel has been limited by their relative discretion, foreign residents say.

East European advisers here say the Soviets rarely even speak to them.

The traditional antipathy between some East Europeans and the Soviets sometimes explodes when Vietnamese yell lien so , a term for Russian, at any foreigner they see.

Whenever that happens, one East European says, he points a finger at his accuser and angrily shouts back in Vietnames, "You Chinese!"