Secretary of State Cyrus Vance yesterday announced that he is undertaking a new peace mission to the Middle East following two days of Egyptian-Israeli talks which he said have laid the basis for a resumption of full scale negotiations.

In a press conference after the close of the talks in a heavily guarded medieval castle, Vance said there had been no progress, in the usual sense of that word, in closing the wide gap between the two nations.

Nevertheless, he protrayed the "candid and probing" discussions here as a significant step toward the resumption of the negotiations which were broken off by Egypt six months ago. Vance said he expects to convene another joint meeting of the Egyptian and Israeli foreign ministers when he travels to the Middle East in about two weeks. Diplomatic sources said the conference may be enlarged to include defense ministers and military experts.

Vance's statements were made in a solo appearance before reporters and only on his behalf. The lack of a joint statement or joint appearance with Egyptian Foreign Minister Mohammed Kamel and Israeli Foreign Minister Moshe Dayan was a sign of the "major differences" which Vance said remain between the Middle East neighbors.

In a separate statement, Dayan said the castle discussions created "much better understanding" of each other's positions and said they were "the first real ones in which we had talks for hours and hours." Israeli sources said Dayan told Vance that Israel will take part in the new meetings which the United States plans.

Kamel was more reserved, saying that "there was no movement but at least they have our plans and can take them back." President Anwar Sadat, who was quoted Tuesday as saying further talks are useless without a new element or a response from Israel to Egyptian proposals, reportedly has not yet agreed to the new conference. But few observers expect him to turn down a formal request by Vance.

There was no indication where the next round of talks might be held, other than the statement that it will be in the Middle East. There has been speculation about the desert site of El Arish in the Sinai or Alexandria on Egyptian coast.

U.S. special envoy Alfred Atherton is heading to the Middle East, Vance announced, to prepare for the next round of meetings and to maintain contacts. Atherton plans to visit Jordan and Saudi Arabia in addition to Egypt and Israel, and Vance said he would "not be surprised" to make similar stops on his own forthcoming tour.

The secretary of state said his plan is to "help in getting things started" rather than to remain in the Middle East for shuttle diplomacy or some other extended diplomatic enterprise. He would give no etimate of when major progress might be made or a peace agreement concluded, but said "I believe Egypt and Israel remain fully committed to establishing a genuine peace."

Vance had planned a Middle East trip this summer even before the start of the talks behind the guards and moat of ancient Leeds Castle, where the British government moved the meeting after a reported terrorist threat in downtown London.He decided to go ahead with the trip when the castle meetings fulfilled their minimum objective of deepening the dialogue between the two countries on their differing proposals for the future of the West Bank and Gaza Strip, the major stumbling block to an Egyptian-Israeli pact.

If agreement is reached to restart fullscale negotiations on the basis of the "self-rule" plan for the West Bank and Gaza Strip submitted last December by Israeli Prime Minister Menachem Begin and the Egyptian counter-plan submitted early this month, it will mark the third major approach by the Carter administration to comprehensive Middle East settlement.

The first approach, from inauguration day to November, 1977, called for convening a Geneva peace conference with the participation of Egypt, Syria, Jordan as well as Israel. This foundered on the question of Palestinian representation as well as the difficulty of the substantive issues involved.

The second approach began with Sadat's dramatic trip to Jerusalem last November and was based on negotiation of a "statement of principles" between Egypt and Israel covering the West Bank-Gaza Strip and the Palestinian question, as a step toward and area-wide agreement. This approach was stymied by fundamental differences, Sadat's withdrawal from the Jerusalem negotiations in January and Begin's refusal on a trip to Washington in March to move toward agreement on the principle of the withdrawal.

The genesis of the new approach was Dayan's trip to Washington late in April to spell out Begin's "selfrule" proposals in greater detail while bypassing the broad question of a "declaration of principles."

The United States convinced Egypt to present a counterplan so that discussions could focus - as they have this week - on the implementation of the two sides' ideas for the West Bank - Gaza Strip and Palestinian questions.'

The United States hopes that by approaching a meeting of minds on details, Egypt and Israel will be able at some point to formulate a statement of principles which could lead eventually to an area wide settlement. This detailed approach is more difficult in several respects than starting with broad and ambiguous principles, but at least it may keep the dialogue going for a time with the hope that pressures to reach compromises can be generated.