Secretary of State Cyrus R. Vance will attend the inauguration of Ecuador's new president next week and plans to use the occasion for an informal summit discussion with Latin American leaders on Nicaragua and unrest in other parts of Central America.

Reliable sources said yesterday that Vance will join First Lady Rosalynn Carter, who will represent her husband at the inaugural of Jaime Roldos, in Quito on Thursday. The sources said Vance will return to Washington Saturday.

According to the sources, Vance decided to make the trip because the occasion, which will mark Ecuador's transition from military to civilian rule, will be attended by presidents and other top officials from several democratic Latin American countries.

The sources said Vance is especially interested in discussing economic reconstruction aid for Nicaragua with the leaders of oil-rich Venezuela and its partners in the five-nation Andean Pact of South America.

Under Venezuela's leadership, the pact members played a major role in efforts to force the ouster of former Nicaraguan president Anastasio Somoza. On a visit to Venezuela and other countries of the region last week, Viron P. Vaky, assistant secretary for inter-American affairs, reportedly found them anxious to discuss ways of aiding the new Sandinista-dominated government in Nicaragua in rebuilding that strife-torn country.

In addition, the sources said, Vance wants to sound out the views of Latin Leaders about how to deal with the potential spill-over of turmoil from the Nicaraguan revolution into the neiboring military-dominated countries of El Salvador, Guatemala and Honduras.

Concern that Cuba, which provided training and arms to the Sandinistas in their fight against Somoza, might now try to foment accelerated guerrilla warfjre in these countire has triggered a debate within the Carter administration about resuming U.S. military aid to Guatemala and El Salvador. Such aid was alted two years ago.

Defense Department and CIA officials, fearful of spreading Cuban influence in the region, have argued for a resumption of military assistance. But the State Department, which wants to put the emphasis of U.S. policy on trying to establish good relations with the Sandinistas in Nicaragua, has advocated a go-slow approach in drawing closer to the region's rightist military regimes.

Reliable sources said the question was debated anew at a White House meeting on Thursday, with both sides essentially sticking to their positions. The sources said the meeting ended in a decision to continue holding the military-aid proposal in abeyance, but to keep it under continual review as a policy option.

According to the sources, at least one more meeting to discuss arms and training assistance to El Salvador and Guatemala is scheduled to be held at the White House next week under the auspices of the National Security Council.