Secretary of State Cyrus R. Vance arrived here today for Tuesday's delayed opening of Egyptian-Israeli talks buoyed by tangible proof of the key American role in the Middle East peace negotiations.

Fresh from solving a major Egyptian-Israeli agenda wrangle -- which held up the opening of the talks for 24 hours -- Vance said upon arrival that the "United States will participate actively" in the search for a comprehensive Middle East settlement.

symptomatic of the re-emerging U.S. role -- which less than a month ago was discounted in the euphoria of the first Egyptian-Israeli meetings -- was Israeli Foreign Minister Moshe Dayan's statement that "Israel needs the United States as an active mediator."

Egyptian realization of dependence on the United States has been clear since the Dec. 25 summit conference between Egyptian President Anwar Sadat and Israeli Prime Minister Menahem Begin failed to produce instant breakthrough. President Carter interrupted his world tour two weeks ago to visit Sadat in Aswan, Egypt, and pledge the United States would "play a very strong role" in Jerusalem.

In his arrival statement Vance said the meeting starting at 11 a.m. Tuesday with the Egyptian and Israeli foreign ministers "shows how far the parties have come in breaking down the barriers that impeded negotiations" in the past.

But a senior American official aboard the plane bringing Vance from Washington stressed that the talks would be long and hard and involve tough bargaining.He warned against expecting any visible results before Vance's expected departure Friday for Washington, with a stopover in Cairo to see Sadat.

Excessive optimism or pessimism were out of place, the official said, and the United States would actively try to bridge differences in the talks, which could be expected to have ups and downs.

But the official said Vance would consider his visit here successful if by Friday the conference had held serious discussions on overall principles and on the guidelines for the future of the Israeli-occupied West Bank and Gaza Strip, the most troublesome issue.

It was American formulations that broke the logjam on the key points Sunday after Vance at the last minute postponed his scheduled departure from Washington Saturday night.

Vance met with Begin for nearly two hours tonight and gave the Israeli leader a letter from President Carter, according to State Department spokesman Hodding Carter III. The contents of the letter were not made public.

In a briefing with Israeli reporters today, Dayan was at pains to put the best possible gloss on the American role in producing an agenda text widely interpreted as more sympathetic to Egyptian than to Israeli wishes.

Dayan insisted that Israel made no major concessions, denied any American pressure was involved and described the United States role as that of honest broker.

He conceded some points of disagreement existed between the United States and Israel -- an apparent allusion to President Carter's recent reiterated support for Palestinian "legitimate rights" and criticism of Israeili settlements in occupied Arab land.

According to authoritative Israeli accounts, these are the points of the agenda:

1. A declaration of principles to serve as a basis for a comprehensive Middle East settlement.

2. Guidelines for negotiations on the issues of the West Bank (called Judea and Samaria in the Israeli version) and the territory of Gaza.

3. Discussion on the components of a peace treaty between Israel and its neighbors on the basis of U.N. Security Council resolution 242, which calls for Israel's withdrawal from the occupied territories.

Analysis of the new agenda suggests that Israel no longer insisted that Begin's controversial "self-rule" plan for the West Bank and the Gaza Strip figure as a separate entry.

Egypt prevailed in having as the first agenda item the declaration of principles -- the most important of which in Egyptian eyes involves selfdetermination for the Palestinians.

Previously, Israel had insisted that the item on the nature of peace --involving normalization of relations as well as formal peace treaty provisions -- take precedence.

In turn, Egypt dropped its insistence that the Palestian question itself -- and the right to self-determination -- figure as a separate agenda heading.

Dayan told reporters that "Egypt cannot put a pistol to our head by threatening to walk out" of the negotiations, although observers fully expect Egypt to seek occasional suspensions of the talks to focus world opinion on its case.

Morsi Saad el Dine, the Egyptian spokesman, refused to comment on Dayan's remarks to avoid an escalation of recriminations.

In Cairo, however, criticism of Israel and warnings about the precariousness of the peace talks continued. Banner headlines in Cairo's three government-controlled dailies said the talks faced a "grave crisis" that threatened chances of success, UPI reported.

[Egyptian Premier Mamdouh Salem accused the Israeli government of "placing obstacles in the way of peace," and said that if this continues, "the time will come when Israel will be confronted by a different situation as a result of its failure to understand the meaning of peace."]

Meanwhile, interest centred on Egyptian and Israeli appreciations of the compromise American plan for a declaration of principles involving the prickly Palestinian question.

Central to American thinking are negotiations, during an initial five-year transistion period, on the eventual status of the Israeli-occupied West Bank and Gaaz Strip.

The American plan sidesteps both Israel's refusal to accept an independent Palestinian state and Egypt's insistence that the West Bankers and Gazans exercise their right to self-determination in a way that could not rule out eventual independence.

Whether such an obvious postponement of the core issue would entice other Arab states -- especially Jordan, which Washington, Cairo and Jerusalem want to link with any Pelestinian entity -- to join the peace talks remains to be seen.

Without such participation Sadat would be hard-pressed to defuse his Arab critics' charges that from the start he was not interested in an acceptable comprehensive settlement, but only in a separate peace with Israel.