Defense Secretary Harold Brown said yesterday that he doubts that the United States and the Soviet Union will sign a comoprehensive arms control agreement by October but that there is "a pretty good chance" for a modest pact.

Brown said at a Pentagon news conference that the "more modest" pact he had had in mind could be either an "explicit" or "tacit" agreement to abide by the limits on strategic arms set forth in the Vladivostok understanding of 1974.

President Ford and Soviet party leader Leonid I Brexhnev at that time agreed that each side would build no more than 2,400 strategic missiles and bombers, with the total number of multiple warheads (called MIRV) limited to 1,320.

The earlier, basic agreement, which effect on Oct. 3, 1972, was to remain in effect for five years to give U.S. and Soviet negotiators time to work out a more comprehensive strategic arms limits of 1974 were to form the basis for a new agreement.

President carter in March submited two proposals to the Soviets, one along the lines of the Vladviostok understanding and the other a deeper cut in the number of strategic weapons each side has deployed. Soviet leaders, in rejecting Carter's comprehensive proposal, complained it would freeze them into strategic inferiority.

"The probablities of a comprehensive agreement are not high," Brown said, but "there is a pretty good chance that that there will be some sort of agreement" by Oct. 3, 1977, of a more modest kind."

One of the biggest sticking points in negotiations to date has been American cruise missiles, short-winged drones that could carry either conventional or nuclear warheads. U.S. cruise missiles are much more accurate than their Soviet counterparts.

Soviet leaders have sought to limit the ranges of cruise missiles on both sides to 600 kilometers, or 372 miles, but Carter in his comprehensive of 2,500 kilometers, or 1,550 miles, for strategic cruise missiles.

Asked yesterday whether cruise missiles were "vital" to the American arsenal and thus could not be banned under an arms agreement, Browm replied that "the cruise missile is very important from a military point of view" but has been overestimated by some people who have praised it as the key new military development of the century."

In other comments at his news conference and in a brief meeting with reporters afterward, Brown discussed these other topics:

- Korea - He said he will visit Korea later this year in connection with implementing Carter's promise to withdraw American ground troops over a period of time.

- NATO purchases - The United States should buy "a greater percentage" of its military equipment from European nations, Brown said.

- B-1 bomber - In making his recommendation this month on whether to put the bomber into production, Brown said the key question will be what kind of Soviet defenses would confront the $100 million plane in the 1980s. - Soviet "beam" weapon - The Aviation Week & Space Technology magazine report that the Soviet "may soon" have an energy beam weapon to destroy American missiles "has no basis in available evidence," Brown said."I have seen all the evidence there is . . . I don't think that there is such a weapons system in prospect in the foreseeable future." He said the same thing is true of laser beam weapons.