As the waitress serves tea, the disco hit "Ring My Bell" blares from two stereo speakers. On the dingy walls of the ca-phe (pronounced like cafe), as it is called here, are posters of the groups Abba and Boney M. In one corner is a shelf holding a Buddhist altar and in another a stack of Asahi and Heineken beer cans. Between them is the ubiquitous portrait of Ho Chi Minh.

The clients include a couple of youths wearing jeans and long hair, and a woman who sits on the doorstep to have her scalp deloused.

Such is the ambiance in one of the private cafes that have been flourishing in this communist capital since authorities began relaxing restrictions on small-scale capitalism and western influences--such as disco music--about six months ago.

According to western diplomats here, such incongruous scenes exemplify the sway that Vietnam's economic pragmatists currently hold as they pursue reforms that are slowly improving the country's economy. But the slight cultural loosening in rigidly austere Hanoi may also reflect influences percolating up from the southern half of the country that fell to the communists in 1975, some observers believe.

"The reason it is going better here is the influence from the south," one diplomat said bluntly.

Some even read the changes as a sign that Hanoi has all but given up its efforts to bring the recalcitrant south into ideological line and that, instead, the conquerors are taking lessons from the vanquished.

Trains from Hanoi to the south are usually chockablock wih passengers who return laden with goods ranging from medicines sent from abroad to copies of Levis jeans manufactured in Ho Chi Minh City, or Saigon as its inhabitants still call it.

PRIVATE ENTERPRISE also thrives at the Dac San, a small converted apartment billed as the only French restaurant in Hanoi.

Run by ethnic Chinese, the place has been a uniquely tolerated fixture since 1974, and the owner claims never to have had any trouble with the authorities. He concedes, though, that the price of that tolerance is rising, with taxes more than doubled in the past two years.

Tucked away on Ly Quoc Su Street in downtown Hanoi, the place is reached by walking down a long, foul-smelling corridor and climbing a steep flight of concrete stairs. Proprietor Nguyen Van Ngoc, 33, who wears collar-length hair and a moustache, is on hand to greet patrons.

He says he gets about 40 customers a day, about 60 percent of them Vietnamese, and grosses about 80,000 dong a month ($8,250 at the official exchange rate and a tenth of that at the black market rate). Taxes, which the Finance Ministry collects every month, have gone up from 7,000 dong a month in 1981 to 18,000 this year, he says.

A three-course meals costs about 400 dong. A bottle of 1978 beaujolais, the only wine available, runs an extra 600 dong, more than double an average factory worker's basic monthly salary.

The menu boasts turtle soup, bouillabaisse, French-style bread and pate, plus steaks billed as chateaubriand and tournedos that one cynic termed "the best water buffalo I've ever tasted."

THE VIETNAMESE authorities have not yet come around to allowing cinemas to screen the cheap westerns and police thrillers that draw crowds in so many Third World countries. But they seem to be coming close.

Near Hanoi's market recently a theater was showing an improbable double feature. One poster, portraying a cowboy on a horse and another strumming a guitar, advertised an East German western. The title: "Let's Sing as We Round Up the Cattle." Also on the bill was a local crime movie, with a poster of a man aiming a gun at the viewer.

Farther down the street, the free market offers more scenes of clashing cultures. Old women, their gums and remaining teeth stained bright red from chewing betel nut, sell flowers, fruit and joss sticks. Other stalls display T-shirts with "Levis" or the names of western rock groups written prominently across them.

Away from the bustle of the market, beyond the mausoleum of Ho Chi Minh and the walls of the central prison formerly known to U.S. inmates as the Hanoi Hilton, a more tranquil mood prevails. Cyclists glide down tree-lined boulevards past decaying but still majestic buildings of the French colonial era. One of the more ornate is a copy of the Paris Opera, a centerpiece of the city that the French built to be their capital of Indochina.

Considered one of the most beautiful cities in Southeast Asia, Hanoi radiates a serene charm complemented by its dozens of placid lakes. Yet reminders of the Vietnam War and its uneasy aftermath abound in the plaques that commemorate the downing of American planes and the small bomb shelters that line some streets. Sometimes the contrasts are sharper, more immediate.

Beside one of the lakes recently, near the old Den Quan Thanh Pagoda, a one-legged man in a military jacket hobbled along on crutches. Behind him, on the edge of an islet, antiaircraft guns pointed up silently over the untroubled waters.