"I have always felt that here was the U.N. and we should try to use if constructively," Secretary of State Cyrus Vance told his American colleagues here last night. "I think if we do try to use it, the U.N. could become an effective forum."

Vance's remark illuminates a change in Washington's attitude toward the world organization following the years of benign neglect to which it had been subjected by the Nixon and Ford administrations.

After two weeks of intensive bilateral consultations during which Vance met with almost 80 foreign ministers, the secretary and his advisers are known to feel "terribly pleased" with the outcome.

The United Nations, in their view, has performed a "useful function" by providing an umbrella under which it was possible to engage in complex Middle East negotiations, discuss the Cyprus problem, the Ethiopian-Somali war, Rhodesia and efforts to limit the U.S. Soviet naval presence in the Indian Ocean and to hold meetings with officials of several countries with whom contact would otherwise be difficult.

In the late category, for instance, fall a gesture of good will toward Vietnam, whose envoy was invited to a lunch President Carter gave Wednesday for Asian delegates.

Whether this renewed American attention will reverse the decline of the United Nations' influence and whether accords reached here this week will eventually be translated into reality are open questions, however.

One U.S. initiative fizzled out before it ever got off the ground. Andrew Young, the U.S. ambassador to the United Nations, proposed a ceasefire in the Ogaden conflict and a U.N. supervised referendum to give the Ogaden population a choice between Ethiopia and Somalia.

The reaction of the Somali foreign minister, Abdurahman Jama Barre, was described as noncommital bafflement. His country has been engaged since July in a war with Ethiopia over Ogaden, first by proxy and later by a direct troop involvement. Ethiopia's foreign minister, Feleke Gedle-Ghiorgis, raised strenuous objections. Emerging from a meeting with Vance yesterday, the Ethiopian told reporters that Vance had repudiated the idea.

"There can't be any plebiscite in Ogaden," the Ethiopian minister said, visibly annoyed. His country has lost nearly two-thirds of the vast desert region to the Somalis since last July. July.

On the Rhodesian issue, however, the Anglo-American plan for black majority rule was given Security Council blessing when the council agreed to the appointment of a U.N. representative to assist in working out a cease-fire in the Rhodesian guerrilla war. The Chinese or the Soviets could have torpedoed the entire plan by blocking in the Security Council this first phase of the Western initiative.

But developments on the thorny problems of Cyprus and especially the Middle East over the coming months will ultimately determine whether the administration's new approach will succeed. Political observers here were far more cautious on these issues than top State Department officials, who yesterday appeared to be in a self-gratulatory mood.

The crucial aspect of the Cyprus problem thus far has been the inability and perhaps unwillingness of Turkey, which occupied two-thirds of the island after a 1974 invasion, to enter into substantive negotiations on the future shape of the Cyprus federation and to relinquish parts of the occupied territory.

Cyprus has created tensions in Washington's relations with Greece and Turkey, including a congressional embargo on arms deliveries to Turkey and the closing by Turkey of U.S. bases in that country.

Both Carter and Vance have been involved in discussion of Cyprus and the foreign ministers of Greece and Turkey have held bilateral consulations on the problems that divide them. Vance and former Defense Secretary Clark Clifford have held two lengthy sessions with Turkish Foreign Minister Ihsan Caglayangil but no details were disclosed by either side.

Greek and Greek Cypriot officials, however, said they were very encouraged by the Carter administration's approach. According to these sources, Carter had assured Cyprus President Spiros Kyprianou that the United States intends to press strongly for the resolution of the Cyprus question.

"The Turks figure that if the United States can push Israel as they did this past week, they are likely to feel far less constraint in pushing Turkey," one diplomat commented.

The Middle East issue, however, remains the most difficult problem. There are still crucial issues to be resolved if reconvening of the Geneva conference is to take place in December.

With the parties to the conference still sharply disagreeing over such fundamental issues as final borders, the Palestinian homeland question and the very nature of peace, the American push for Geneva seems intended to create a forum for protracted discussions between Israel and the Arabs.

At this stage, at least on the basis of comments made by the parties involved, there seems to be no assurance that the Geneva conference would succeed.

The risks involved in the venture include the possibility that in the absence of assured success, the conference would collapse, with disastrous consequences. What top administration officials are hoping - although they are not saying it publicly - is that the Geneva conference will be held in session for months and perhaps years, giving the participants the opportunity to nibble away at the problems confronting them and ultimately creating a basis for a lasting settlement.

As one senior American official put if, "three or four weeks ago there were many people who said we couldn't get a unified Arab delegation at Geneva. Now we have agreement on that. And that is a big step because it accepts the concept that there should be Palestinian voice at Geneva. The question is, who are those Palestinians? How do you organize working groups? But even there, we found the beginning of flexibility."