Former secretary of state Cyrus R. Vance yesterday endorsed research on President Reagan's proposals for a space-based missile defense system but added that he is "strongly opposed" to pursuing the so-called Strategic Defense Initiative beyond the current level of research.

"Technically, a leak-proof defense is an unrealizable dream," Vance said in criticism of administration suggestions that the SDI, which has been dubbed "Star Wars," could provide a shield over the United States that would change the emphasis in nuclear warfare from offensive to defensive weapons.

Vance told the Senate Foreign Relations Committee that attempts to develop and deploy an antisatellite system of space weapons would cost "hundreds of billions of dollars" and show no evidence that such a plan would succeed.

Instead, he argued, undue concentration on the SDI would become an obstacle to reaching agreement with the Soviet Union on reducing the intercontinental strategic nuclear missiles that now are the principal weapons of the superpowers.

"Before we begin to modify today's strategic doctrines, we must be absolutely certain that what we put in their place will not make nuclear war more likely," he said. "It would be folly, at this time, to abandon an approach that has helped to keep the peace in favor of one that seems, at first glance, to be less morally repugnant, if the upshot would be to increase the chances of nucelar Armageddon. There is no moral advantage in that."

Vance, who served as former president Jimmy Carter's secretary of state for 3 1/2 years, appeared before the committee as part of its comprehensive review of overall U.S. foreign policy. The thrust of his testimony was to call for greater cooperation between Congress and the executive branch in charting the U.S. course in world affairs, but he was largely critical of President Reagan's principal policy goals.

In addition to the SDI, he criticized Reagan's plans to develop the MX missile because, Vance said, it would not intimidate the Soviets but push them to develop new missile technology, thus working against successful arms control negotiations. Instead of the MX, Vance said, the United States should develop the so-called Midgetman missile as an alternative because it is mobile, accurate and has a single warhead rather than 10.

Vance also took exception to calls by Secretary of State George P. Shultz, and other administration officials for a reexamination of the War Powers Resolution, which they claim weakens the credibility of the president's ability to use U.S. military power to deal with foreign crises.

Instead, Vance argued, the War Powers Resolution should be strengthened to provide regular, mandatory consultations with Congress in times of emergency. In an apparent reference to past clashes between the executive and Congress over the use of American troops in Vietnam and Lebanon, he said:

"If Congress were really in on the takeoffs, we would not have the problems we have seen at the crash landings."

Vance criticized the administration's "constructive engagement" policy of friendliness toward South Africa's white-minority government on the grounds that it is insensitive to the feelings of blacks. But he opposed the idea of forcing U.S. companies out of South Africa and said the United States should seek first to use its diplomatic pressure to help the South African business community work toward elimination of that country's racial restrictions.