Heartened by the tentative warming of relations between Vietnam and the United States, U.S. companies are stepping up a campaign against the strict trade embargo that the U.S. government slapped on the Southeast Asian country after the communist victory there in 1975.

In visits and letters to policy makers and legislators, business groups are arguing that the political justifications for the embargo have largely vanished. In any case, they say, it is having no significant effect because European and Asian companies are moving into Vietnam in growing numbers.

"It's another example of America seeking to further foreign policy interests by penalizing American companies and at the same time not achieving the foreign-policy goals," said William Archey, vice president for international affairs at the U.S. Chamber of Commerce.

For now, their arguments are falling on deaf ears in the Bush administration, which is cautiously expanding contacts with Vietnam after 15 years of near total freeze. The White House remains convinced that a stick is a better means than a carrot to achieve U.S. goals in the region.

Last week, it allowed Vietnamese Foreign Minister Nguyen Co Thach, who was visiting New York on United Nations business, to come to Washington for consultations. But it would not let him spend the night here. The countries have no diplomatic relations.

U.S. companies, nevertheless, continue to test the limits of the embargo. Some have got into trouble: In 1989, an American travel agency was fined for organizing tours to Vietnam. But Cable News Network earlier this year won federal approval to exchange video footage by satellite with Vietnamese state television.

American Telephone & Telegraph Co., eager for customers among the 700,000 Vietnamese-born residents of the United States, has twice sent teams to Vietnam and concluded a long-distance phone-service deal with the government there. To avoid sending money to Vietnam, which would violate the embargo, AT&T proposes to pay satellite and earth-station fees on Vietnam's behalf and put remaining money due to the country into escrow pending an end to the embargo.

The U.S. State Department has not approved the proposal, however.

Other U.S. corporate delegations continue to visit Vietnam to prepare for the time the embargo is removed. Under U.S. law, they can make contacts and gather information but cannot sign deals or do business. Among the visitors this year: teams from Mobil Oil Corp., discoverer in the early 1970s of oil deposits now being developed with European help, Chevron Corp. and Texaco Inc.

"The embargo should be revisited," said Dave Dickson, a Texaco spokesman.

The companies seeking change want first of all to make money. But their arguments also mark a new chapter in a debate over the proper U.S. foreign policy for an era in which the country can no longer assume economic predominance. The United States must give more thought to the impact that geopolitical decisions have on American companies and the U.S. economy, critics said.

"It's very clear that the battlefield of the '90s is an economic one," said Archey of the U.S. Chamber of Commerce, which recently helped organize a "Vietnam normalization coalition" of about 25 American companies and a half-dozen trade associations. Because of the continuing political sensitivity of the issue, most companies involved in the Vietnam debate do not want to be publicly identified, leading to the role the chamber has taken on their behalf.

Some businesses also argue that U.S. trade will help speed reform in the communist country. "We believe American business can be a potent force for change in Vietnam," John Kamm, president of the American Chamber of Commerce said in a letter sent to members of Congress this month.

Their efforts have won some sympathy on Capitol Hill. Sen. Richard Lugar (R-Ind.), a Foreign Relations Committee member, is circulating a draft joint letter to the White House calling for liberalization of trade provided that Vietnam continues to cooperate with the United States on key issues.

But the administration argues that such talk is premature. Despite progress, key issues remain on the table between the two countries, it said, citing efforts to end the civil war in Cambodia and to account for the more than 1,600 Americans listed as missing-in-action during the Vietnam War.

The United States is willing to normalize relations with Vietnam, a State Department official said, but only in the context of an overall settlement of these questions. The fact that Vietnam has actively courted U.S. companies shows that the embargo still has meaning, he suggested.

Even when the door is opened, no one expects a bonanza. Exhausted by decades of war and mismanagement, Vietnam remains one of the world's poorest and most technically backward countries. Its 67 million people also face the additional challenge of a phaseout of the Soviet aid that has helped keep it afloat.

Yet some analysts believe that if it stays with free-market reforms it has begun, Vietnam could attract significant foreign investment, expand its $2 billion in annual two-way trade and emerge in the long run as a solid economic performer, as have many of its neighbors in Southeast Asia. It is sometimes compared to China 10 years ago.

The U.S. embargo dates to 1975, when communist forces from the north overran South Vietnam two years after the withdrawal of American combat troops. U.S. resolve to keep the embargo was strengthened in 1978 when Vietnam invaded its neighbor Cambodia to overthrow the brutal Khmer Rouge government there. Khmer Rouge and non-Communist guerrilla factions have been fighting the government that Vietnam installed ever since.

Now Vietnam has withdrawn most of its forces from Cambodia and is cooperating in a U.N.-sponsored plan to organize elections there. It has promised new help to the United States in tracing the missing soldiers. Both actions are cited by critics of the embargo as reasons to drop the trade ban.

Many Western countries still maintain sanctions against Vietnam. But most interpret them more liberally than does the United States, particularly in regard to oil.

European oil companies that include Petrofina SA, Compagnie Francaise des Petroles (Total) and British Petroleum PLC have signed deals to develop offshore deposits. Also present in the country, according to the American Chamber of Commerce in Hong Kong, are Japanese trading companies, French banks and textile makers from Singapore and Canada.

As of March of this year, according to the Hong Kong chamber, Vietnam had reported signing 140 foreign investment deals that together are worth at least $1 billion.