President Carter's planned Camp David summit meeting with Anwar Sadat and Menachem Begin has abruptly shifted the focus of Middle East peace efforts out of this region and back onto the controversial role of the United States in pressing both sides to agree to peace.

The shift came with a swiftness that stunned even the man who arranged it, Secretary of State Cyrus R. Vance. Seated in a lawn chair at Egyptian President Sadat's summer palace here Monday night, Vance personalized the invitation from Carter by reading it aloud, then settled back expecting to hear Sadat ask for time to consider it. Instead, he accepted immediately. Israeli Prime Minister Begin had already secretly agreed in Jerusalem.

American officials acknowledge that the Sept. 5 summit could be anything from a bandaid to a break-through. At the least, they feel they have helped Sadat and other Arab moderate leaders buy some more time, a goal that has become an over-riding one for the Carter administration. The outcome of the new, high-profile American involvement will be determined in large part by the Carter administration's interpretation of several key diplomatic code words that Sadat uses and that Vance embraced warmly Tuesday night at a joint press conference.

Vance forcefully asserted that the United States would accept Sadat's call to become "a full partner" in the negotiations and would present its own ideas for compromise at the summit if they would help break deadlocks. Sadat, who has frequently said that peace will come only when the United States pressures Israel into giving up occupied Arab territories, said on July 30 that the United States had to act "as a full partner, not a mediator" now that his own dramatic personal initiative to the Israelis had run out of steam.

Wary of the Carter administration's intentions, Israel insists that the U.S. role remain that of mediation and opposes the presentation of "American peace plans." A high-level American official said here last night the Sadat and Vance were in agreement on what the phrase "full partner" means, but declined to clear up the evident contradition between that judgment and Vance's press conference assurance that the Israelis "are delighted that we are playing the role."

The secretary also bolstered Sadat's case for climbing down from his demands for new Israeli concessions before talks resume by calling U.N. Resolution 242 "the basic underlying document and principle on which the search for peace is based." He added that the summit meeting would try to find "a framework within Resolution 242" for a final peace agreement.

Israel disputes the Carter administration's interpretation that Resolution 242 calls for substantial withdrawal from the West Bank territory, which Begin calls by the biblical names of Judea and Samaria. This conflict could be the major problem for the summit.

The senior American official indicated in a briefing to reporters last night that the essential objective of the summit will be to get agreement on a declaration of principles for peace negotiations. The Sadat initiative, launched by the Egyptian president during his visit to Jerusalem last November, began to falter in January when the two sides could not agree on such a declaration.

But the State Department does not consider that the peace search is starting over again from zero. Discussions by Vance and the Egyptian and Israeli foreign ministers last month is England, as well as Israeli security needs and Arab views on territory encouraged the view that the time may be nearing for American proposals that would not be called a peace plan but that would put a heavy American imprint on the process. The idea of a three-nation summit at Camp David had been proposed inside the administration before the London Conference and Sadat's angry breaking off of direct contacts with Israel on July 30. Sadat's move added urgency and at the same time raised the odds against Vance's highly secretive carrying of the invitations to the Middle East.

The secretary considered his trip, a long-shot effort, the U.S. official said. The signals coming into Washington were that Sadat felt vulnerable to rising Arab criticism and domestic opposition to his initiative. But the administration had come to the conclusion that is fundamental policy of helping maintain Sadat, the Saudi Arabian royal family, and other moderate Arab leaders was in peril from rising impatience with the Sadat initiative. President Carter decided that the risk had to be taken.

Only two of Vance's aides knew of the invitations when the American party left Washington. Special envoy Alfred Atherton, who has been shuttling between Egypt and Israel since March, and the American ambassadors in those countries were kept in the dark until Vance arrived in the region. Neither Sadat nor Begin had been advised that the invitations were coming.

Vance saw Begin alone Sunday morning and without consulting his Cabinet or hearing in detail what Vance expected to tell Sadat, the Israeli prime minister happily accepted the chance to resume personal negotiations with the Egyptian president.

Foreign Minister Moshe Dayan was not told of the invitation until three hours after it had been extended, according to one account, and may even have been unaware of Begin's acceptance until Vance left, according to another account. The U.S. official said secrecy was needed so that neither leader would feel he was being pressured into coming to Camp David. Another American source said that there was a fear that leaks would cause suspicions either Begin or Sadat to be suspicious that the United States was plotting to gang up with the other against him.

Reporers traveling with the U.S. party were consistently steered away from the idea that a summit was the number one option at this point. Officials repeatedly said nothing new was taken to the Middle East by Vance and the summit idea was called hypothetical, one of many options under consideration.

The original scenario called for Vance to report back to the president before any announcement was made. But Sadat's immediate acceptance caused the White House to jump the gun, the U.S. official said, and to rush out an announcement Tuesday before the news could be leaked by either the Egyptians or Israelis.