Secretary of State Cyrus R. Vance said yesterday there are signs of "not unpleasant" changes in the Soviet military presence in Cuba, but he cautioned reporters not to "leap to optimistic conclusions" about the status of Soviet forces on the island.

At a press conference dominated by questions about tensions in the Caribbean, Vance sought to downplay the importance of new Soviet naval construction in Cuba and hinted that Moscow may be trying to reduce U.S. concerns about the Soviet combat forces stationed there.

Initially, he refued to give details except to say, "Some factors are difference from what they were." When reporters pressed him for specifics, he said "It is not unpleasant," but added the warning about drawing too optimistic an interference from his remarks.

Senior Carter administration sources later said privately Vance had not meant to imply that there has been any detectable reduction in the combat brigade of 2,000 to 3,000 men that the United States claims has been put into Cuba by the Soviets.

Instead, the sources said, Vance was referring to U.S. intelligence surveillance that has detected a lowered level of activity by the Soviet forces. As one source put it: "There's no sign of a reduction. But we are seeing much less traffic and physical movement than we saw before."

The brigade initially was detected by U.S. intelligence monitoring in late August because it apparently was engaging in large-scale maneuvers at the time. The sources said its current decreased activity is regarded as potentially significant because it could represent a Soviet effort to demonstrate that its mission is to train Cuban troops and not to engage in activities posing a threat to Western Hemisphere nations.

The controversy provoked by discovery of the brigade threatened for a time to cause a major confrontation between Washington and Moscow. It led President Carter to announce in a televised address Oct. 1 a number of measures designed to counter Soviet and Cuban activities in the Caribbean area.

But, in talking about the Cuban situation yesterday, Vance took an almost determinedly low-key approach, saying, "We are trying to keep everything in its proper proportions."

In regard to another aspect of the Cuba controversy, Vance confirmed reports that the Soviets are constructing a second pier and some large buildings at the Cuban naval base on Cienfuegos Bay. That has prompted speculation about whether the Soviets may try to use the base to service warships, including submarines with a nuclear capability to strike at the United States.

He said, "There is no indication the pier is for anything other than Cuban use." While he conceded that the construction might lead to port calls at Cienfuegos by Soviet warships, Vance said such calls do not violate the U.S.-Soviet agreements barring introduction of offensive weapons into Cuba. u

Vance also admitted that one of the buildings under construction at Cienfuegos is similar to those used by the Soviets elsewhere for storage and repair of nuclear missiles. But, he noted, the Soviets also have used these buildings for other purposes such as storage of small, conventional vessels.

He also took the unusual step of using the press conference to take issue with a report by columnist Jack Anderson. The columnist had asserted that the State Department and Carter's national security affairs adviser, Zbigniew Brzezinski, had clashed over the wisdom of using the Soviet brigade issue as the basis for a propaganda campaign designed to discredit the Soviets and Cubans with other countries.

According to the Anderson column, Brzezinski, over the opposition of Vance, had ordered them to provide data on Cuban activities. Anderson said two-thirds of the ambassadors protested the instructions.

In response, Vance said, "We have been concerned about the activities of the Cubans and the Soviets in various parts of the world. We have conveyed our concerns to other countries and have kept them up to date on our thinking, our views and our information with respect to these issues."

He then added that the messages to collect this information and to convey it to other governments were sent "at my instruction."

Privately, administration sources, while conceding there have been differences between Brzezinski and the State Department about the wisdom of too much emphasis on propaganda attacks on Moscow and Havana, said the degree of these differences had been exaggerated in the Anderson column.

These sources confirmed Vance's statement that the instructions to ambassadors and the relaying of messages to other governments had been drawn up and implemented by the State Department.

They also characterized as totally incorrect the assertion that two-thirds of the ambassadors had protested the policy. Only a few, scattered protests had been registered by U.S. ambassadors around the world, they said.