Secretary of State Cyrus Vance said today he is prepared to undertake a shuttle mission between Cairo and Jerusalem if that would help to break the deadlock impeding agreement on an Egyptian-Israeli peace treaty.

Vance acknowledged the possibility of a shuttle effort while answering questions following a speech he made here to the Royal Institute of International Affairs.

After the long-scheduled speech, Vance will leave Sunday for Egypt and then Israel in an attempt to overcome the differences between Egyptian President Anwar Sadat and Israeli Prime Minister Menachem Begin.

Originally, Vance tentatively had announced plans for stops of only one or two days in each country and had said he planned to return to Washington by the middle of next week.

However, in response to questions today, he conceded that it might not be possible to get the U.S.-mediated talks on a peace treaty concluded by the Dec. 17 date set forth in the Camp David agreements and indicated that he might have to spend more time in the Middle East to get the negotiations back on the track.

"If it appears that a shuttling process would be helpful, I am prepared to do that," Vance said. He added, though, that this would be a matter for the Egyptians and Israelis to decide.

He noted that the treaty negotiations, which began Oct. 12 in Washington, have been stalled by disagreements over two principal issues: Israel's reluctance to accept Egyptian demands for a timetable on negotiations leading to Palestinian autonomy in the Israeli-occupied West Bank and Gaza Strip and Egypt's unwillingness to accept language stating that the peace accord with Israel must take precedence over Egyptian defense pacts with other Arab countries.

Echoing the concern expressed by President Carter Thursday, Vance said: "I do not believe that peace should be denied to the people of the Middle East by a failure to resolve these problems."

The questions about the Middle East situation tended to overshadow the main point of Vance's speech -- a discussion of U.S. policy toward Europe that included a strong attempt to reassure Maerica's European partners that a U.S.-Soviet strategic arms limitations agreement "will strengthen the security of the United States and our allies."

State Department officials described the speech as a major attempt to clarify the Carter administration's European policies before a European audience. While conceding that the speech broke no new ground in terms of policy initiatives, the officials said its intent was to sketch Washington's views on the entire range of strategic and economic issues in which the United States and Western Europe have common interests.

In talking about defense questions, Vance put special stress on a phrase that lately has been used with increasing frequency in administration circles -- "strategic nuclear parity between the United States and the Soviet Union."

That involves the concept that neither of the two superpowers can hope any longer to gain a clear-cut edge over the other in nuclear weaponry and must seek instead to neutralize each other as a military threat by maintaining roughly equal strengths.

"The fact of strategic parity remains," Vance said. "Just as we will match the Soviet increases, so we must assume the Soviets are re solved to match us. Thus, the pursuit of superiority by either side would result in frustration, waste, increased tension and -- in the end -- reduced security for all."

He then sought to put the current U.S.-Soviet strategic arms limitation talks (SALT) into this framework. In this way, Vance appeared to be addressing the concern of boyh European and domestic U.S. critics that a SALT II agreement, by neutralizing the U.S. and Soviet nuclear deterrence capabilities, would alter the strategic balance in Europe and make America's NATO allies more vulnerable to conventional forms of military pressure.

"@salt ii/ will not rule out the force programs we have under way to meet the challenges that will remain even with an agreement," Vance asserted. "We have preserved all our major strategic force options. Other programs that can strengthen deterrence in NATO can go forward. Allied interests have been protected."

"That is why we are involved in SALT -- because a sound agreement will improve Western and global security," he added. "A good agreement can provide more security with lower risk and cost. And we recognize that without SALT the strategic competition could infect the entire East-West political relationship, damaging the effort to create a less dangerous world which is at the heart of Western foreign policies."

He conceded, though, that a SALT agreement, to be effective, must be linked to other strategic measures by the Western alliance.Among them, he said, are the need for NATO to continue improving its ability to respond to Soviet nuclear forces targeted against Europe, continued improvement of NATO's conventional forces through such steps as the current drive to get member countries to boost their defense budget by 3 percent and moving ofrward with additional arms control initiatives such as the mutual and balanced force reduction (MBFR) talks on thinning out East-West conventional forces in Central Europe.

In that part of his speech devoted to economic issues, Vance cited President Carter's inflation-fighting program and the recently signed energy legislation as steps taken to fulfill the commitments made by Carter at the seven-nation economic summit meeting in Bonn last July.

These moves, he asserted, underscore the administration's determination to deal with the complaints of West European governments that the weakness of the dollar and heavy U.S. imports of oil are having disruptive effects on the economies of their countries.

He also called for "the successful completion -- this month -- of the multilateral trade negotiations" as a means of ensuring sustained economic growth for both the United States and Europe.

But, he warned, a successful conclusion of the Geneva negotiations must not be undermined by "piecemeal retreats towards protectionism which could undermine that progress."