Secretary of State Cyrus R. Vance yesterday announced that a Soviet combat brigade has been secretly deployed in Cuba for several years, at least, and that the United States is demanding that something be done to correct the newly revealed situation.

In a news conference and a closed-door appearance before the Senate Foreign Relations Committee, Vance would not specify what the United States will demand from the Soviets to ease this newest cause of superpower discord. He suggested that, at a minimum, Washington is seeking a statement of the "purpose and intention" of the 2,000 to 3,000 heavily armed troops.

Asked whether Washington will insist on removal of the troops, Vance replied cautiously, "I will not be satisfied with maintenance of the status quo." Officials said the U.S. demand is being kept vague to create the maximum chance for successful negotiations with Moscow.

The United States took the matter to the Soviets late last week by calling Soviet Deputy Ambassador Vladillen M. Vasev to the State Department. Vasev returned to see Vance in an unannounced meeting yesterday, presumably with a preliminary reply from Moscow.

Intensive negotiations are expected to begin when Soviet Ambassador Anatoliv F. Dobrynin returns to Washington Friday. Dobrynin has been delayed in his return from home leave by the death of his father, sources said.

"We regard this as a very serious matter affecting our relations with the Soviet Union," Vance told reporters in a statement worked out at a meeting of top White House advisers late Tuesday.

He said the presence of the Soviet combat unit in Cuba "runs counter to long-held American policies," but said it was not covered by U.S.-Soviet understandings about Cuba in the Kennedy and Nixon administrations, and added that the brigade lacks the airlift or sealift to give it an "assault capability" against the United States.

A statement by Vance, speaking for the high councils of the Carter administration on this military and political hot potato, had been widely expected at yesterday's news conference. The most surprising part of his remarks, and the part that drew persistent questioning from reporters, was the claim that the Soviet troop unit has been in Cuba, undetected, since "at least the mid-1970s," before President Carter took office.

Senior officials were embarrassed and, by their account, surprised by intelligence confirming the Soviet combat force just as the Senate moves into the decisive phase of its debate on the strategic arms limitation treaty (SALT II).

Despite much skepticism by outsiders, the officials insisted that the timetable of the Soviet troop deployment had not been adjusted to predate the responsibility of the Carter administration, and they continued to deny that timing of the revelation about the troops was adjusted to embarrass Cuba during the current conference of non-aligned nations in Havana.

Vance said re-analysis of "fragmentary" intelligence in the light of new information suggested that elements of a Soviet brigade might even have been in Cuba since the early 1970s "and possibly before that."

Some officials suggested that the Soviet combat unit might even be a remnant of the forces sent to Cuba by Moscow before the 1962 U.S.-Soviet confrontation over strategic missiles on the island.

U.S. intelligence has known for many years that several thousand Soviet military men were present in Cuba, but not in a combat role. The Russians have been assumed to be military trainers and advisers and operators of a very large and important communications interception station, according to officials.

By early 1976 at the latest, U.S. intelligence received indications that some of the Soviet troops in Cuba were organized as a combat unit, the sources said. There were additional "occasional references" to a Soviet brigade over the months since then, they added.

These indications and references, according to the official account, were inconclusive and were not taken at face value by the intelligence community as a whole.

A National Security Agency analyst who completed a retrospective study this June is credited with the conclusion that a Soviet brigade organization existed in Cuba, but not necessarily an active brigade with troops and equipment.

This finding set off higher priority intelligence-collection activities and an intensive review within the administration, officials said.

By mid-July several members of Congress had heard of the intelligence findings, and one of them, Sen. Richard Stone (D-Fla.), began raising a public alarm.

After considerable internal dissension, a top-level review in late July reported that "intelligence does not warrant the conclusion that there are any other significant Soviet forces in Cuba" apart from the long-observed Soviet advisory force. This finding was transmitted in summary form to Stone in a letter signed by Vance.

At the same time, however, intelligence sources who had lost the battle for official endorsement were saying there were firm indications, gathered over the previous six months, that Soviet combat troops were stationed in Cuba. The troops were reported to be organized as a brigade in classic configuration involving 1,200 to 1,600 men equipped with tanks, personnel carriers and artillery.

This description is similar to Vance's statement yesterday that the Soviet brigade includes "motorized rifle battalions, tank and artillery battalions and combat and service support units."

A crucial piece of intelligence that changed the official assessment was gathered Aug. 17, according to informed sources. At that time, the United States photographed Soviet troops in maneuvers in circumstances that left no doubt of their identity or nature.

High-level officials have little explanation of why U.S. agents in the Soviet Union, Cuba or elsewhere did not come up with a credible report of a sizable Soviet combat unit on the Caribbean island for at least 3 1/2 years and perhaps as long as 17 years. The only answer being given is that Cuba is an extremely inhospitable place for intelligence-gathering, as CIA Director Stansfield Turner told the Senate committee yesterday.

In response to questions, Vance denied that the Carter administration's goodwill gesture of January 1977, calling off U.S. spy plane flights over Cuba, contributed to the lack of information. He laid stress on "the final piece or two [of intelligence] which puts the jigsaw together" as the critical factor in the reassessment.

Vance was asked if Washington might respond to the Soviet forces by reinforcing U.S. units at Guantanamo Bay, where they occupy Cuban territory despite the opposition of the Castro regime. He refused to say what actions the United States might take to counter the Soviet forces.

At the White House, top foreign policy advisers huddled on the Cuba question yesterday for the second day in a row. Officials said President Carter had formally approved a set of recommendations about the Soviet troops issue that was informally presented to him late Tuesday by national security adviser Zbigniew Brzezinski after a policy review meeting of high-level officials.

The officials would not describe the recommendations, other than to say that Vance's public statements were part of them.

Vance said yesterday that the mission of the Soviet combat unit in Cuba is unclear. There is a wide range of conjecture about the mission among intelligence officials. Among the possibilities discussed: to protect sensitive Soviet installations; tropical training; as a core group for force projections into the Latin region; as a defensive unit in case of a U.S. threat to Cuba; to protect Soviet nuclear weapons in case these should be introduced into Cuba.

While denying that the U.S. announcement of the Soviet brigade was timed to embarrass Cuba as it plays host to nonaligned nations, Vance went out of his way to attack Cuban President Fidel Castro's leadership in the Havana meetings.

When reporters failed to ask at his news conference, Vance later issued as a State Department release a charge that Castro is campaigning "to shift the nonaligned movement away from genuine independence and into a pattern of collaboration with the Soviet Union."

Vance rejected a direct link between the Soviet troops and the pending SALT pact, saying SALT hearings should continue. But his cautious response and mention of overall U.S.-Soviet relations left room for an effect of the treaty.