Secretary of State Alexander M. Haig Jr. today accused Vietnam and the Soviet Union of undermining the stability of Southeast Asia through their support of a "puppet regime" in Cambodia.

Haig spoke on the opening day of a U.N.-sponsored conference of 77 countries called to develop a strategy for getting Vietnam to pull its troops out of Cambodia, where they have forcibly installed a government friendly to Hanoi and Moscow.

Haig said Vietnam's "seizure" of Cambodia in December 1978, its installation of a "puppet regime" there and its continued occupation with about 200,000 troops directly threatens the security of Thailand, a U.S. ally, and "undermines the stability" of all Southeast Asia. He called the Soviet Union "the financier of the Vietnamese military occupation."

The work of the five-day conference being held in the U.N. General Assembly is hampered, however, because Vietnam, the Soviet Union and 24 other countries, many of them aligned with Moscow or Hanoi, have refused to participate.

Thus, while the purpose is to try to find "a comprehensive political settlement" of the Cambodian problem in which "all conflicting parties participate," there will be no real debate here. Virtually all the countries taking part were among the clear majority of the United Nations who voted for U.N. resolutions in 1979 and 1980 calling for such a conference, for the total withdrawal of foreign troops from Cambodia and for U.N.-supervised free elections.

Hanoi has made it clear that in its view any U.N.-backed plan put forward by five-member Association of Southeast Asian Nations (ASEAN) and supported by the United States and China would be only an attempt to replace the current Cambodian government of Heng Samrin, which was installed by Hanoi, with an anti-Vietnames government. Any such government, Hanoi contends, would depend on Washington and Peking for its security and upon the united Nations and Western donors for economic aid.

Singapore's foreign minister, S. Dhanabalan, said states attending the conference are not interested in prolonging the conflict in Cambodia "in order to bleed Vietnam." The high cost of Vietnam's military occupation of Cambodia is generally viewed as being largely responsible for Hanoi's economic plight.

In a more conciliatory tone than his government has previously used, the minister said Southeast Asian states would like to see an "independent, prosperous Vietnam that was not the victim of superpower conflict."

Despite the apparent stalemate in the international effort to persuade Vietnam to withdraw its forces, officials from the United States and many other countries say there is value in this conference for several reasons.

As they see it, the meeting keeps world pressure on Vietnam. It dramatizes the costs to Hanoi of its military occupation and growing international isolation. It attempts to show that Vietnam is occupying Cambodia for expansionist reasons rather claims. Finally, delegates here hope to develop a more specific strategy for finding a negotiated solution.

The ASEAN nations -- Indonesia, Malaysia, the Philippines, Singapore force behind the conference.They are drafting a resoluton that, in addition to a troop withdrawal and elections, will call for a U.N. peacekeeping force and refer to the need to disarm all forces in the country as a prelude to free elections, according to ASEAN sources. It will also refer to possible economic aid to Cambodia and perhaps "other countries," which might include Vietnam.

The last two points could become the most debatable issues at the meeting.

China, which supports the Pol pot government Hanoi overthrew, opposes disarming the 40,000-man guerrillas force loyal to Pol Pot that is fighting Hanoi's forces. China also resists economic aid to Vietnam.

The suggestion of aid is seen by many diplomats here as necessary to get Hanoi to adopt a softer line toward negotiations. Hanoi acknowledges that the war has strained its economy and U.S. officials say Moscow is sending $3 million to $6 million a day to keep Vietnam afloat.

State Department officials have said privately that while the United States has no plans to offer Vietnam aid before a troop withdrawal, a shift in policy by Hanoi could change "all kinds of things."

Haig said today that the united States will "continue to question seriously any economic assistance to Vietnam -- from whatever sources -- as long as Vietnam continues to squander its scarce resources on aggression." Some observers saw this as an indication U.S. aid would follow a Hanoi withdrawal.

The united States already gives economic aid indirectly to Vietnam through the U.N. Development Program. Last month, according to U.N. officials, this group decided to give another $118 million to Hanoi during the next five years.

Haig said the United States sees the conference as having two goals: restoration of Cambodian soveregnty and "a neutral" Cambodia that represents no threat to any of its neighbors.

Pol Pot, whose opposition forces are the strongest and who is backed by China, is a dictator whose former Khmer Rouge regime is viewed by many as among the most brutal in recent history. Some Western analysts have estimated that between 1 million and 3 million Cambodians starved to death or were killed while Pol Pot was in power.

While the current Hanoi-backed government is not represented here, leng Sary, a representative of Pol Pot's "democratic Kampuchea" spoke to the assembly and said Vietnam wished to use Cambodia as a "springboard" to use Cambodia as a "springboard" to the rest of Southeast Asia