Secretary of State Cyrus R. Vance quickly welcomed Soviet President Leonid I. Brezhnev's offer yesterday to stop peaceful nuclear testing, along with weapons test, as "a major step" toward halting all nuclear explosions.

The Soviet decision to forgo "so-called peaceful nuclear explosions." Vance told a news conference, "is in the direction of what we have been talking about for several months" to achieve "a comprehensive test ban."

Brezhnev's offer still leaves some questions on closing off the last area - underground - where the two nations conduct nuclear blasts.

The Soviet leader proposed "a moratorium" all peaceful tests, as distinct from "a ban on all nuclear weapons tests for a definite period." The United States wants a flat ban on both, with a review within "four to five years." The Soviet Union reportedly wants a limit of about three years.

The Soviet action is "an important step" in seeking "a downturn in the arms race," Vance said. Athough Vance did not mention it, a total U.S.-Soviet test ban would overtake two treaties now pending before an unenthusiastic Congress to limit nuclear weapons and peaceful tests to 150 kilotons (150,000 tons of TNT equivalent).

Vance made no immediate comment on Brezhnev's other proposal to halt production "by all states" of "atomic, hydrogen or neutron bombs or missiles." Such sweeping arms proposals are regarded by U.S. officials, who have made some sweeping proposals of their own in past years as postures rather than practical negotiating offers.

Overall, Vance told reporters, "I think that there has been an improvement in relationships between ourselves and the Soviet Union in the last several weeks, and I think this is positive."

He acknowledge, however, that the Carter administration privately has raised with the Soviet Union, "with great seriousness." American concern that some leading Soviet dissidents will be tried on charges of working for U.S. intelligence agencies.

That was one of the issue raised by Vance in a meeting Monday with Soviet Ambassador Anatolly F. Dobrynin. President Carter previously has expressed concern about Soviet prosecution of a prominent dissident. Anatoly Scharansky, alleged to have worked for the Central Intelligence Agency, which Carter denied.

Vance said, "We have discussed the question of the possibility of the trials with the Soviet Union." He declined to discuss details.

Vance said that "the relationships between ourselves and the Soviet Union are always a mixed set of factors," with "currents and cross-currents" in different directions. But the recent overall trend, he said, is toward improvement.

There is no "linkage," Vance said, between Soviet treatment of dissidents and U.S.-Soviet negotiations on nuclear arms control. Unlike President Carter, Vance declined to project any timetable for concluding the current strategic arms limitation talks (SALT). He said the negotiations are "too complicated" to speculate on that.

Vance officially confirmed yesterday that the United States is "withdrawing" an American naval attache from South Africa, and also is "recalling" a commercial officer from that country in the American review of economic relations with South Africa.

U.S. actions in retaliation for "the regrettable" South African crackdown on black groups and their supporters, Vance said, will include a formal U.S. ban on "all export of items for police and military in South Africa," plus military spare parts and maintenance equipment, as previously indicated by U.S. officials.

The ban includes such items as computers and executive aircraft, in the "gray area" beyond the U.S. voluntary arms embargo on South Africa since 1933.

On Monday in the United Nations, the United States, joined by Britain and France, expressed willingness to vote in the Security Council for a mandatory ban on arms shipments to South Africa, but vetoed African demands for stronger economic and other sanctions against South Africa.

Vance said the United States wants to hold open to South Africa "the importance of beginning progress toward the end of apartheid" (separation of races), and yet "reflect our national concern" that it has taken "a major step backward."

Strong criticism of the Carter administration for refusing to take sterner action against South Africa came yesterday from Americans for Democratic Action. The political group, declaring it was acting "with great reluctance" because of the Carter administration's record on African affairs said that it "deplores the decision" to veto "punitive sanctions" against South Africa in the United Nations. ADA said South Africa's actions "clearly show that it is a threat to world peace . . . "

The Vance news conference, his first in Washington since July 29, produced an unusually strong warning about the international danger of another rise in world oil prices.

"I think it would be catastrophic," Vance said, "if there should be an increase at this point in energy prices."

Such an increase, Vance said, would be economically unjustified, and "would set us back very substantially because of the possibility of increased inflation and increased unemployment . . . "

Vance discussed at some length American negotiations with the Arab nations and Israel to produce a Middle East peace conference in Geneva, but added no new details.

Vance totally rejected, as President Carter did last night in addressing the World Jewish Congress, charges that the Carter administration is sacriticing Israel's interests to achieve an Arab-Israeli settlement.

" . . . We are not playing "Russian roulette'" with the Geneva conference, as charged on Tuesday by Senate Minority Leader Howard H. Baker Jr. (R-Tenn.), Vance said.

On the contrary, Vance said, the U.S. Soviet declaration on Oct. 1 of principles for a Geneva conference "was a constuctive step which helped to move us toward the convening of a Geneva conference.

In responding to Israeli and American Jewish apprehensions about U.S. policy, Vance said. "I want to emphasize that we are committed to the security of Israel, and there has never been a moment of doubt upon this."

In negotiations, Vance said, there has to be "flexibility and a willingness to work out differences between the parties," and "therefore, there is the possibility of change . . ." That "obviously raises in the minds of all the parties questions and concerns, but one of the biggest problems the United States has between Israel and the Arabs "is our attempt to overcome this mistrust which exists on both sides." CAPTION: Picture 1, Secretary Vance: "I think that there has been an improvement in relationships . . . " [WORDS ILLEGIBLE];Picture 2, Soviet President Brezhnev addresses the Supreme Soviet in a speech marking the 60th anniversary of the Bolshevik Revolution. Tass via AP