Soviet Ambassador Anatoliy F. Dobrynin is scheduled to meet today with Secretary of State Cyrus R. Vance in the first head-to-head negotiation between Soviet and U.S. representatives over the presence of a brigade of Soviet troops in Cuba.

Dobrynin, who returned yesterday from a vacation and consultations with Soviet officials in Russia, is expected to make known for the first time the Soviet position on the 2,000 to 3,000 combat troops stationed in Cuba.

A State Department spokesman declined to say yesterday what stance Vance would take when he meets the Soviet ambassador at the State Department. On Friday, President Carter said the situation required "firm diplomacy" and that the "status quo is not acceptable."

In private conversations last week, Vance made clear that he intends to keep a low public profile on the negotiations to maximize the chances for success.

State Department officials have indicated that a hard line in public on the presence of the Soviet troops 90 miles from the U.S. coast would almost surely cause the talks to fail. The issue requires delicate face-saving on both sides, officials have said.

Soviet specialists in the State Department have indicated that the absence of a formal declaration up to now by the Russians could be a good sign.

State Department officials requested an early meeting with Dobrynin on Aug. 29, the day before news of the brigade in Cuba was made public. Dobrynin has been out of Washington for several weeks. Last week, the ambassador's father died in Russia.

In a television interview yesterday, Senate Foreign Relations Committee Chairman Frank Church (D-Idaho) cautioned the Soviets to take the troop issue seriously.

On "Face the Nation" (CBS, WDVM), Church said, "What I am interested in now is that these negotiations begin on a serious note and not an attempt to dismiss it." He noted that the Soviet news agency Tass had brushed off U.S. concern about the Soviet combat brigade.

Church reiterated his feeling that unless the combat troops are pulled out of Cuba, there is no likelihood of Senate approval of the strategic arms limitation treaty (SALT II) that the United States and the Soviet Union have signed.

He said that the Soviets had invited "a period of testing" with the United States when they stationed the combat brigade in Cuba.

"I think that time has come," Church said.

President Carter, Church said, has "plenty of leverage" to exert pressure on the Soviets to remove the troops. Church suggested trade pressures or deployment of U.S. troops elsewhere in the world as possibilities, but said he would not second-guess Carter on what measures to take.

The Senate Foreign Relations Committee first raised the possibility of Soviet combat troops in Cuba during a July 13 executive session, Church said. "At the time it was a rumor," he said.

Church said neither Defense Secretary Harold Brown nor intelligence officials at the meeting could verify the report, and the committee issued a statement denying the story.

The Washington Post said yesterday that the first hard evidence of the troops' presence came from an Aug. 17 overflight of a U.S. spy satellite.

Church announced the news of the troops Aug. 30 after he said he received indications from the State Department that the Carter administration did not plan to make the information public. But, said Church, "they indicated this would surface in the press within 24 to 48 hours."