Secretary of State Cyrus R. Vance said yesterday that the Egyptian-Israeli talks opening in Cairo next week have supplanted a Geneva conference as the prime forum for peace in the Middle East.

Vance acknowledged at a news conference that events have overtaken the Carter administration's drive for starting peace talks at Geneva between Israel and all its Arab adversaries. The process that "could have started at Geneva," he said, "can also start in the Cairo discussions in the circumstances that now exist."

"Now," he said, "I think that we should seize any opportunity and we should not concern ourselves about the forum in which the peace process starts . . ."

Egyptian President Anwar Sadat and Israeli Prime Minister Menahem Begin, Vance said, "have made a breakthrough, and we intend to help them wherever possible to enlarge that opening."

Vance intends to carry that message to the leaders of the Middle East in six days of talks starting next weekend. He is scheduled to leave Washington early this morning for a conference of the North Atlantic Treaty Organization in Brussels. Following the NATO meeting, Vance will visit Egypt, Israel, Lebanon, Syria, Jordan and Saudi Arabia.

Only the United States and the United Nations, in an observer capacity, have agreed to join Egypt and Israel in the Cairo conference launched by Sadat after his dramatic visit to Jerusalem last month.

The Declared purpose of the Cairo meeting is "preparing the conference in Geneva and ensuring its success with the purpose of achieving an overall settlement of th Middle East conflict . . ." But the entire diplomatic pattern has been reshaped by Sadat's initiatives. The "informal meeting in Cairo" has overtaken the intended Geneva conference, split Arab ranks wide open, and put the United States and the Soviet Union on opposing sides of the Cairo meeting, although they are the designated co-chairmen of the intended Geneva conference.

This turnabout has been assailed in the Soviet press as "a plot" by "imperialism and Zionism and Arab reaction" against the Soviet Union's friends in the Arab World - including Syria and the Palestine Liberation Organization, who have declined to attend the Cairo conference. In Cairo, the Soviet press has charged, Israel, the United States and Egypt intend "to camuflage a separate deal" between Egypt and Israel.

Secretary Vance, who rarely makes any critical comments about the Soviet Union, said yesterday, "Some of the statements which they have made in recent days have not been helpful; they raise questions . . . about what their ultimate objectives are."

"We still believe," Vance said, "that the Soviet "ultimate objective is to see a comprehensive settlement of the Middle East problem, and to work as co-chairman to that end." But Vance said he will await a report in Brussels from Under Secretary of State Philip C. Habib, who ended talks in Moscow yesterday.

Sen. Frank Church (D-Idaho), who presided over a closed Foreign Relations Committee hearing on the Middle East, said afterward:

"I would say that the curtain has gone up on a new stage in the Middle East on which the scenery has been arranged by Sadat and Begin. I don't think anyone can predict what kind of a play wil unfold on that stage, but the United States suddenly finds itself a spectator, or, at most, relegated to a supporting east - which may be the best role for us at this time."

Assistant Secretary of State Alfred L. Atherton Jr., who will represent the United States in the Cairo talks, told the Senate committee that American policy there will be "to be helpful," as Egypt and Israel desire.

Vance said he expects to see real movement toward peacce out of the Cairo talks. "I would hope," he said, "that out of the Cairo conference it might be possible to come up with a framework of the substantive matters that have to be dealt with in order to achieve a comprehensive agreement; and in addition to that, the remaining procedural questions might be cleared away."

Vance said "the time has come . . . to really begin to come to grips with the question of substance." This includes, he noted, agreeing upon the nature of peace between Israel and all its border adversaries, Egypt, Jordan, Syria and Lebanon; Israeli withdrawals from war-occupied territory, and Aran Palestinian demands for a homeland.

Egypt's Monday decision to sever diplomatic relations with five severe critics of its diplomacy - Syria, Iraq, Libya, Algeria and South Yemen - evoked only a mild reaction from Vance. "I think it was too bad that President Sadat was forced to break relations with the five Arab nations," Vance said. He expressed hope that "in time those differences may be healed, particularly insofar as Syria is concerned . . ." Syria, which is important in U.S. peace strategy, agreed yesterday to receive Vance on his trip.

Asked when a Geneva peace conference, long projected for this month, might ultimately convene. Vance said he could not guess at a date "and I don't think it is that important. I think the important thing now is to see how much progress we can make at the Cairo conference. That is what is on the table now."

The United States, Vance said, may be able to help keep communication channels open, where they are blocked now, hoping that in time "the circle of those discussing the peace process will be enlarged again."