The 32d U.N. General Assembly opened today with the election of Vietnam into full membership.

What might have been an occasion full fo the symbols of healing and rapprochement, however, was marked by one more example of the confrontations that loom as the likely themes of the three-month-long session.

The opening ceremonies were delayed for two hours by a dispute over the voting rights of the Cambodian delegation, which ended only with a temporary agreement that no voting at all would take place for an indefinite period while negotiations continue.

As a result, the assembly elected Yugoslav Deputy Foreign Minister Lazar Mojsov as its president by acclamation, without voting, and accepted Djibouti and Vietnam as its 148th and 149th members under the same procedure.

The dispute arose when the Cambodians refused to pay their U.N. dues for two years - a total of $36,984 - on the ground that this was a debt of the previous regime of Lon Nol.

Under the U.N. Charter, any nation in arrears for two years is ineligible to vote. This principle, applied to the Soviet Union in 1964 over nonpayment of peacekeeping assessments, caused the assembly to go through an entire year without voting until the United States agreed to drop the issue.

Since then, other nations failing to produce the necessary funds have voluntarily refrained from voting until they paid up. But the Cambodians refused to accept this practice.

The assembly is likely to get by without a formal vote on the election of its other officers and the adoption of its agenda, both scheduled for later this week. But eventually it must come to grips with more controversial issues, such as southern Africa or the Middle East, on which decisions by acclamation will not be possible.

Mojsov, 56, a career diplomat who served as ambassador to the United Nations from 1969 to 1974, spoke of the "stagnation and wavering" that have blocked the paths to the solution of international crises, creating "new hotbeds of conflict."

He warned that the time for transforming the awareness of global change into action is growing short, because inaction is fast leading toward "universal catastrophe." But he expressed doubt that the present U.N. session would depart from the past practice of inaction.

Speeches welcoming Vietnam and Djibouti were so numerous that they spilled over into Wednesday's schedule.

Tonight's last speaker was U.S. Ambassador Andrew Young, who, on behalf of the Carter administration, said simply that the United States hopes that it and Vietnam can build a "constructive relationship" at the United Nations.

Then, in his personal capacity, Young said Vietnam's own struggle had been accompanied by a struggle within the United States to end the war, and that struggle was led by his mentor, the late Rev. Martin Luther King Jr., among others. King's widow, Coretta, was present as an American delegate to the current assembly session.

Young ended by expressing the hope that Vietnam's entry "is a step in the struggle for peace, justice and prosperity which we now carry ontogether in the U.N. and in our own countries."