A jubilant Prime Minister Menachem Begin yesterday welcomed the forthcoming Middle East summit, terming it "a chance well worth taking."

Interrupting a week's vacation, which had begun only this morning, Begin emphasized at a press conference that Israel had not been asked by Secretary of State Cyrus Vance to make any changes in its peace plan. Nor had any proposals been made by Israel that differed from its previous position, he said.

Begin also said he had not received any hint from Vance that the United States might present its own ideas for a Middle East peace at the Camp David meeting.

"This has been the American position all along, and I see no reason why this course should be abandoned at this point," the Israeli prime minister added.

Earlier, Begin sent a message to Vance in Alexandria congratulating him for his "outstanding success" in bringing about the summit meeting.

"President Sadat and I owe a vote of thanks to President Carter for his initiative in setting the peace process in motion again," Begin said in his message.

At last night's press conference, Begin disclosed that from Sunday morning until noon yesterday, he had been the only person in Israel aware of the new Carter initiative.

He said Vance had given him a handwritten message from President Carter and asked him to read it on the spot. Halfway through it, said Begin, he came to the actual invitation. When he finished reading, he said he told the secretary of state without hesitation that he accepted.

Begin said he could do so without further consultaion because the step was in accordance with previous decisions taken by Israeli government. Begin then sent the president a message coded in a way that it could be understood only by those who had seen Carter's invitation.

Asked whether the Camp David summit might pose a danger to Israel should the talks end in a deadlock, Begin said, any part of the peace process entails dangers. But, he added, the summit is a risk well worth taking.

Begin's visible optimism seemed to be shared by most Israeli political leaders. The head of the opposition Labor Party, Shimon Peres, one of Begin's severest critics, last night joined in the general praise for the Carter move.

"It is a rare opportunity of extreme importance," Peres said.

Peres congratulated Begin for the renewed opportunity to further the cause of peace, but warned that the forthcoming meeting will be difficult since both sides come to it with same basic points of disagreement.

Other Israeli observers also stressed the danger in the deep gap that presently separate Israel and Egypt. They suggested that even if President Carter does not present American proposals, his position as host and chairman at Camp David will constitute an implied pressure on Israel.

While Begin obviously understands this risk, observers feel his enthusiastic endorsement of the summit was at least in part prompted by a desire to emphasize that his government's position is not "intransigent."

Begin also said last night that as far as he is concerned, the recent personal attacks and insults directed against him from Cairo will play no part in his thinking.

"I shall never forget that Sadat came to Jerusalem," Begin said.