Soviet President Leonid I. Brezhnev has assured the United States that the Soviet Union will not establish military bases in Vietnam, President Carter said today.

The president said Brezhnev made that assurance personally to him last week when the two leaders met in Vienna for three days of talks and the signing of the new strategic arms limitation treaty (SALT).

Carter disclosed that Brezhnev assurance in an interview with Japanese reporters, the text of which was made available here, where the president made a refueling stop en route to Tokyo.

For some time, the administration has been concerned about the Soviet military buildup in Vietnam and particularly that the Soviets might establish permanent military bases there that could be used to threaten other Southeast Asian nations. But in the interview, Carter, citing his conversations with Brezhnev, downplayed that concern.

"We have no fear of the Soviet military presence there," Carter said. "We are concerned about a buildup whenever we consider it to be excessive beyond what is required for Soviet security.

"I discussed the South Vietnamese question with President Brezhnev and particularly the Soviet presence there, both ships and airplanes. He assured me personally that there would be no establishment of Soviet bases in South Vietnam and that the present ship and plane use of the ports and airports is of a routine nature."

The Soviets have made heavy use of the American-built port at Cam Ranh Bay to supply their Vietnamese allies and the fear has been that they would turn that facility into a permanent Soviet naval base.

In what is expected to be a major theme of his trip to the Far East, the president pledged that the U.S. "military involvement in Asia will not be decreased." However, Carter insisted that he has not yet decided whether to go through with his earlier commitment to withdraw American ground troops from South Korea.

"We will maintain American military presence in the western Pacific adequate to protect American interests and to protect the interests of our allied," he said.

In the interview, the president also took a shot at John B. Collanny, an announced Republican presidential candidate. Connally has accused Japan of closing its markets to American goods and said that if he were president he would threaten the Japanese with a trade embargo in retailation.

Asked about this, Carter defended Connally's right to speak out and added: "I would like to point out Mr. Connally holds no public office, and perhaps when you analyze his statement you will see why he holds no public office."

The president and his party stopped for an hour here at Elmendorf Air Force Base. Carter left here late this afternoon for Tokyo, where beginning Monday he will make a state visit to Japan, attend an economic summit conference with U.S. allies and from there go on to South Korea for another state visit.