U.S. Secretary of State Cyrus Vance arrived in Egypt tonight at the start of a tour intended to reassert Washington's role in the Midde East peace-making process.

Vance said that he had come to express support for President Anwar Sadat's peace campaign and would discuss with the Egyptian leader "the ways in which we can best help to move this process forward in support of the action which he has initiated."

Vance is also going to consult Arab leaders opposed to, or less than enthusiastic about, Sadat's initiative in hopes of preventing their exclusion from the process. Also on Vance's agenda is discussion of Moscow's role in the peace-making process.

Before flying to Cairo, Vance told a news conference in Brussels that the Soviet Union is adamantly opposed to Sadat's initiative hut that Moscow still supports cooperation with the United States toward a comprehensive Middle East settlement.

Vance's remarks followed a private report to him by Under Secretary if State Philip Hahib, who had just ended a two-day visit to Moscow in an effort to convince the Soviets not to completely dismiss the Cairo conference called by Sadat. The conference is to begin Wednesday.

Vance rejected Soviet news agency reports suggesting that U.S support of Sadat's initiative was contradictory to an earlier joint U.S. Soviet declaration that pledged effots toward reconvening of the Geneva conference on the Middle East.

Ultimately, Vance said, a comprehensive agreement would have to he consummated in Geneva.

Alluding to Sadat's dramatic visit to Israel, however, Vance said there had been new developments since the joint declaration and "we feel as though we ought to take advantage of the new circumstances to proceed with a Cairo conference. But that does not rule out an ultimate Geneva conference so I don't see the conflict."

During his current tour. Vance will try to persuade Jordan and Syria to join the Cairo talks. Syria had joined several hard-line Arab states in sharply rejecting Sadat's initiative Jordan's position has been ambivalent.

Vance, however, was expected to have a difficult time in persuaking Sadat to accept a Soviet role in the peacemaking process.

Sadat has kept up a barrage of criticism of the Soviets, who he believes are fomenting Arab opposition to his policies.

During Sadat's visit to Israel, the Egyptians said they were upset by the American action in issuing a joint statement with the Soviets Oct. 7 offering Soviet and American guarantees for a peace settlement.

The Egyptians viewed this move was amateurish and said it brough the Soviets back into Middle East diplomacy for no good reason, even though the negotiating premises outlined in the document were acceptable.

Vace is expected to be given the message that while Egypt wants to keep the Soviets out now, American help is most welcome.

The United States is still laying great stress on the need for an overall peace settlement Geneva rather than an agreement just between Israel and Egypt, U.S. officials said they told the Soviets that their view of the Cairo talks as being designed to work out a separate peace runs counter to the stated position of all parties concerned.

Yet, there is not doubt that the Cairo conference has eclipsed the Geneva format and has made it impossible for the Geneva meeting to take place this year, as the Soviets and Americans had publicly agreed it should.

The Soviet Union has declined an invitation to the Cairo conference, as have all the Arab states Sadat asked to attend - Syria, Jordan and Lebanon as well as the Palestine Liberation Organization.

Officials in the Vance party said that the Soviets dismiss the Cairo Conference as unimportant and that Washington must get back on the road to Geneva sooner or later after recognizing that the Cairo conference will not work.

So deep is the animosity between the Soviet Union and Egypt that each believes the other is subverting efforts to reach the goals that both profess to want. It will be a remarkable diplomatic feat if Vance can bring them back together.

The Cairo conference still does not have a clear agenda or cutoff point the Americans say they are prepared to take part for several months if necessary, perhaps with a recess for Christmas.

The Egyptians have reserved hotel rooms for Dec. 14 through 22 but Sadat himself has said the talks could last well beyond the end of the year.

Between now and the beginning of the meeting here, Vance is going to be touring the region trying to get other area leaders to keep an open mind about Sadat's initiative even if they cannot support it.

The difficulties are illustrated by the visit here that Jordan's king Hussein completed this morning. He has said he is sympathetic to Sadat but he also has close ties to Syria and has declined to participate in the Cairo meeting unless the other Arabs do.

Hussein left without issuing any statement and a second scheduled meeting between the two leaders said not take place. The Egyptians are resigned to going ahead in their conterence without any other Arab participant if they have to.

It is on Vance's forthcoming stops in Syria and Saudi Arabia that the Americans say they will look for suggestions of support and try to nurture them. U.S. officials say they do not consider Assad a member of the rejection front of states that met in Libya to again proclaim their denial of Israel's right to exist. They say they think Assad attended the meeting both to show his displeasure with Sadat's move and to try to moderate the rejectionist rhetoric.

According to the Americans. Habib told the Soviets that Washington cannot take the ejectionist approach seriously and that is considers the meeting in Libya irrelevant - exactly the view held by Egypt. The Soviet are said to had hinted that they did not disagree.

The Americans are touching bases with all the parties directly concerned except the Palestinians. After a day of talks here. Vance is to go to Jorusalem. He will also visit Syria, Lebanaon and Saudi Arabia.

It is clear, however that these are consultations and that Vance is not bringing any substantive proposals to any of these nations.