The Soviet Union has taken a significant new step toward completion of a new strategic arms limitation treaty (SALT) despite the uncertainty created by China's invasion of Vietnam and other world events, informed sources disclosed yesterday.

In meetings Tuesday with Secretary of State Cyrus R. Vance and President Carter, the Soviet ambassador in Washington, anatoliy F. Dobrynin, presented new Soviet ideas on the few outstanding issues that substantially narrowed the differences between Soviet and U.S. positions, according to authoritative sources.

Officials of the two countries once again are discussing arrangements for a summit meeting between Carter and Leonid I. Brezhnev, the Soviet leader. However, the Soviets have shown an interest recently in holding that meeting somewhere other than the United States, a potential complication, according to U.S. sources.

Administration officials have said all along that the SALT negotiations were proceeding satisfactorily, but during the last 10 days official American anxiety grew that outside events, particularly the Chinese invasion of Vietnam, had disrupted the SALT process.

The official Soviet news nedial blamed the United States for acquiescing in, if not actually encouraging, China's invasion of the Soviets' ally, Vietnam. Several Soviet specialists inside the Carter administration said up to monitor Soviet compliance with the human rights provisions of the Helsinki agreement on European security.

The new group of protesting American scientists, which revealed its existence for the first time yesterday at a Washington news conference, is called "Scientists for Orlov and Scharansky" (SOS). Its supporters include 13 Nobel laureates, 113 members of the 1,100-member National Academy of Sciences, 18 past or present directors of major scientific laboratories, and 20 past or present presidents of national scientific organizations.

Prof. Paul Flory of Stanford University, a Nobel laureate in chemistry, told yesterday's news conference that several international meetings have been canceled "and many others have suffered from greatly reduced attendance" because of the number of Americans who signed two SOS petitions.

One, signed by 1,750 scientists, pledges each signatory to "withhold all personal cooperation with the Soviet Union" until Orlov and Scharansky are released.

The second, signed by 660 Americans, is less sweeping, but pledges the signatories to oppose any expansion of Soviet-American exchanges, to boycott international conferences in the Soviet Union, and to work against the transfer of high technology or the granting of most-favored-nation trade status to the Soviet Union.

"No action of this character or magnitude has ever been taken by scientists on behalf of their colleagues suffering from oppression and living in other countries," a spokesman for SOS said. "There was no concerted action of this type during the Nazi period, nor during the Stalin purges."

Dr. Morris Pripstein, a physicist at the Berkeley Laboratory of the University of California and an early organizer, said SOS was born last summer at Berkeley when friends of Orlov there decided to try to do something to protest his imprisonment.

Pripstein, a high-energy physicist, said many Americans in that field -- one in which there has been extensive Soviet- American cooperation -- decided that "something had to be done that was different than before." Those who chose to end their personal involvement in exchanges with the Soviets included some who had been extremely active in exchanges, Pripstein said.

The idea of personal protests spread after the much-publicized Scharansky trial, he added.

Another SOS organizer, Prof. Kurt Gottfried of Cornell, said the policy of the group was to "deprive the U.S.S.R. of some of the benefits of American science and technology."

The Carter administration has pursued the cases of Orlov and Scharansky through diplomatic channels, and its officials are pessimistic that the two men will be released soon.