The United States had launched a global campaign to punish Vietnam for its invasion of Cambodia by cutting off development assistance from international institutions, and curtailing some humanitarian aid as well.

The immediate object, according to American, West European and Asian officials involved, is to put pressure on Hanoi to open negotiations on Cambodia and an international conference under U.N. auspices that is due to take place in July.

Washington has already signaled its new punitive policy, both at home and in the U.N. arena.

Earlier this month, it rejected an export permit sought by the central committee of the Mennonite Church to send 250 tons of wheat flour to Vietnam. This was the first such turndown of food exports to Hanoi since 1975, according to officials of a humanitarian agency that deals with aid to Vietnam.

At the United Nations, the United States and the European Economic Community have refused to respond to an appeal from the United Nations Children's Fund -- UNICEF -- to provide milk powder, butter oil and CSM (a high-protein food supplement) as an emergency donation to Vietnam.

Some officials have expressed serious doubts that the U.S. campaign will achieve practical results. In addition, tactical differences have developed between Washington and its allies on how to make the case against Vietnam at meetings of U.N. aid agencies.

Officials both here and in Washington say the Reagan administration has adopted the position that as long as the Vietnamese divert their resources to the occupation of Cambodia, "We question granting any assistance to them."

American representatives will be authorized to state publicly at U.N. meetings that aid to Vietnam "amounts to subsidizing aggression" -- aggression that had been condemned by U.N. General Assembly resolutions -- U.S. officials confirmed.

Diplomats from Asian nations opposed to the Vietnamese occupation of Cambodia generally supported the American idea of checking international aid to Hanoi, but remain reluctant to make the case this bluntly.

"We are loath to set the precedent of linking political acts to the flow of U.N. development aid," said one Asian diplomat. "We would prefer that the U.S. not articulate that reason publicly."

The Asians would prefer to make the case that U.N. funds are going "into a bottomless hole" in Vietnam, with inadequate monitoring, and that future funding of development projects should be "put on hold."

The first international forum targeted by the American-led drive is a meeting of the 48-nation council of the U.N. Development Program, which convenes here June 9.

That gathering must approve aid to Vietnam that would provide between $94 million and $118 million between 1982 and 1986. Currently, the U.N. agency has a portfolio of

Bradford Morse, a former Republican congressman from Massachusetts who has run the development program for six years, admitted that the issue of funding for Vietnam "could be raised" at the June meeting, but "I hadn't heard of it."

The issue has been raised in the past by previous American administrations -- most recently in 1976, Morse noted -- and the Vietnam funding "has always been approved."

One veteran State Department source said privately that although the Reagan administration would like "to kill the [U.N.] development assistance program for Vietnam if it can get the horses, this is virtually impossible."

He noted that the majority of Third World nations on the governing council "are not about to accept the principle that you can cut off funding, so it would be a losing battle."

Officials here and in Washington say that there is no intention of trying to cut funds for Vietnam at a meeting of the governing board of UNICEF that is now going on here. Hanoi gets about $5 million a year in programmed aid from UNICEF, plus emergency food supplies for both Vietnam and Cambodia. Since UNICEF helps out mothers and children, it has always been treated as a sacred cow, even by Congress.

"We have always tried to treat UNICEF as nonpolitical," an American official explained. "Vietnam is not going to arise here."

The emergency aid program for Cambodia is scheduled to end in December, and until then, U.S. policy is to "cut the line as close to humanitarian relief and as far from development assistance as possible, although there are some gray areas," said a Reagan appointee at the U.S. mission.

Even if the Reagan administration is unable to stem the bulk of the aid flow to Vietnam, officials said, it is determined to speak out on the issue.

UNICEF sought the emergency food aid donation for Vietnam after another U.N. agency, the Food and Agricultural Organization, reported that Vietnam needs 384,000 metric tons of food aid this year because of typhoons last fall.

State Department officials, while denying that they asked the Common Market to reject the appeal, expressed skepticism about Hanoi's need for these supplies. They again expressed the view that Hanoi should either turn to Moscow for the food or buy it on the open market with the money being spent on the occupation of Cambodia.