Secretary of State Cyrus Vance, here for the General Assembly of the Organization of American States, pledged the Carter administration's support for "breathing more vitality" into the hemispheric organization.

According to State Department spokesman Tom Reston, Vance indicated U.S. approval of efforts to strengthen the OAS at a working dinner Sunday night with the Andean Pact foreign ministers. The Andean Pact -- made up of Venezuela, Colombia, Ecuador, Peru and Bolivia -- has become increasingly active in hemisphere affairs.

Reston said that U.S. security concerns in the Caribbean were touched on during Sunday night's working dinner. But he again denied that the administration has any plan to form a "grandiose" naval task force to patrol the area, saying instead that the United States primarily will rely on economic assistance to bolster stability in the Caribbean.

The General Assembly opened today.

The five Andean countries, with which the United States has worked closely over the past year, hope to turn the OAS into a more important forum for discussing regional political, economic and social issues.

Vance, who arrived here Saturday evening accompanied by a high-level U.S. delegation, was seriously affected by the city's altitude or more than 12,000 feet and spent most of Sunday in bed.

Although Reston said the secretary of state was feeling better today, it was announced that he plans to return to Washington Tuesday evening, 15 hours earlier than his orginally scheduled departure Wednesday morning.

Before Sunday night's dinner with the Andean Pact foreign ministers, Vance met with Bolivia's new president, Walter Guevara Arze. He presented Guevara with a letter from President Carter expressing the president's "total support" for this country's return to democracy, which began with Guevara's inauguration as president in August after more than a decade of military rule.

Vance met today with the foreign ministers of Colombia and Paraguay and will deliver a speech Tuesday to the OAS General Assembly.

Although the 27-member OAS became somewhat active earlier this year in trying to bring an end to Nicaragua's civil war, its annual General Assembly meetings tend to be devoted more to internal administrative chores and to informal meetings between foreign ministers than to serious discussions of policy affecting the diverse nations that belong to the organization.

Among the decisions facing the OAS this year will be whether to re-elect its current general secretary, Alejandro Orfila of Argentina, for another five-year term. Orfila is engaged in a spirited contest with Ramon Emilio Jimenez, the foreign minister of the Dominican Republic.

The OAS also will consider a report of human rights in a number of its member nations, including El Salvador, Argentina and Chile.

Another important issue before the Assembly is Bolivia's desire for an outlet to the sea through territory captured 100 years ago by Chile. The issue has been placed on the meeting's formal agenda, causing Chilean Foreign Minister Hernan Cubillos to stay away. Chile does not have diplomatic relations with Bolivia because of the emotional territorial dispute.

In a welcoming speech this morning, Guevara also criticized the United States for planning to sell 35,000 tons of tin from the U.S. strategic stockpile, which Bolivia, a major tin producer, fears will deflate the price of tin and further damage the country's shaky economy.

The Carter administration has said it will sell the tin in an orderly manner to avoid a sudden price drop that would affect the Bolivians.