Secretary of State Cyrus R. Vance headed into a potential dispute yesterday when he told Congress the United States will honor the nuclear arms ceilings in the American-Soviet pact, which expires Oct. 3, if the Soviet Union "exercises similar restraint."

At the same time, Soviet Foreign Minister Andrel A. Gromyko met with Vance in an attempt to break up the three-year-old stalemate over a new accord in the strategic arms limitation talks (SALT). These talks continue today in the White House.

There are months of negotiation ahead, at best, before a new pact can be reached to replace the expiring nuclear accord, which is at the core of U.S.-Soviet detente policy, the reduction of tension. A congressional clash now looms over the Carter administration's attempt to avoid legislative action on continuing the existing arms ceilings while negotiations go on for a new agreement.

After the first round of talks with Gromyko on what Vance described as "substantial differences." Vance told reporters the atmosphere was "very good, straightforward, businesslike."

Gromyko, reaching for a metaphor he enjoyed, said, "We have, so to say, waded into the stream, but . . . there are a lot of rocks, and I can't tell you right now when we will wade out of the stream . . . So I would urge you all to display some patience."

The Vance-Gromyko talks, in which White House officials also participated, extended over seven hours yesterday and included a review of attempts to reopen an Arab-Israeli peace conference at Geneva.

The United States and the Soviet Union are co-chairman of the Geneva conference on the Middle East, which met only two days in 1973.

A State Department spokesman said the Soviet Union is expected to issue, along with the United States, a parallel declaration on honoring the current nuclear arms ceilings when the accord runs out in 11 days.

In a letter to Senate Foreign Relations Committee Chairman John J. Sparkman (D-Ala.), Vance said: "Our objective is to maintain the status quo" on arms levels in what is known as the SALT I agreement, which expires Oct. 3, "while the SALT II negotiations are being completed, and to complete the work on a SALT II agreement within the near future."

The United States, he said, intends "to issue a unilateral policy declaration" for that purpose. Although "the Soviets may also issue a statement . . . along the same lines," Vance said, "there would be no agreement limiting strategic offensive weapons" between the two nations after Oct. 4, pending a new accord.

Vance said the administration "considered and rejected the possibility of a joint U.S.-Soviet statement which might have raised the question of whether an international agreement was intended."

What Vance did not add was that the administration wants to avoid a noisy congressional debate in the midst of the negotiations. He told reporters yesterday, "I don't believe congressional approval is necessary."

That contention will be challenged by the Senate Armed Services Subcommittee on Arms Control headed by Henry M. Jackson (D-Wash.), an aide to Jackson said yesterday.

The aide was the administration's action will be attacked as "a circumvention" of the Arms Control and Disarmament Act, which makes any arms limitation subject to a treaty or congressional legislation. This dispute has been brewing for months. It is unclear how much opposition Jackson and other critics of U.S.-Soviet detente policy can rally against the administration.

Sparkman said his Foreign Relations, Committee, which is a friendlier forum for the administration, will hold a hearing on Monday with Paul C. Warnke, head of the Arms Control and Disarmament Agency.

The State Department last night made public a letter from House International Relations Committee Chairman Clement J. Zablocki (D-Wis.), and Rep. William S. Broomfield (Mich.), the committee's ranking Republican, supporting administration policy.

In a letter to Vance, they said: "It is our view that under existing circumstances a unilateral policy declaration along the lines you indicated would not constitute an international agreement" requiring congressional approval.

Vance told reporters at the end of yesterday's talks with Gromyko that he is prepared to talk further with Jackson about the dispute over expiration of the current nuclear accord.

Ultimately, any new arms accord will have to run the gauntlet of Senate criticism as a treaty, and Jackson was the prime challenger when the 1972 pact, which is now set to expire was before the Senate.

Under the expiring accord, the United States was limited to 1.054 intercontinental land-launched ballistic missiles and the Soviet Union to 1.618. In submarine-launched intercontinental missiles, the limit for the United States was 656 weapons and for the Soviet Union 740.

The complex accord, however, also permitted "freedom to mix" in these categories, allowing up to 710 sea-launched missiles for the United States and 950 for the Soviet Union, by switching sea-launched missiles for land-launched missiles.

The United States accepted Soviet advantages in these numbers because the 1972 agreement contained no limit on American superiority in strategic bombers and missiles with multiple warheads.

In 1974 at Vladivostok, an attempt was made to balance off the disparities by putting a ceiling of 2,400 on the numbers of intercontinental missiles (sea-launched or air-launched) on each side of which no more than 1,320 could have multiple warheads.

That accord, between Soviet leader Leonid I. Brezhnev and President Ford, has been the core of subsequent negotiations. The dispute has centered largely on apply the limitations to newly developed American long-range cruise missiles and to a new Soviet bomber known as Backfire.

Cruise missiles - low-flying, pilotless aircraft with great accuracy that can be launched from air, sea or land - are the more potent weapon.

The Carter administration last March collided with the Soviet Union over attempts to drastically revise the Vladivostok accord by major reductions in arms ceiling, controls on the quality of weapons and cutbacks of the massive Soviet land-based missiles.

Last May in Geneva, the United States proposed a three-part compromise for a SALT II accord. It would include a treaty extending until 1985: a three year protocol including limits on cruise missiles, Soviet Backfire bombers and Soviet SS-18 missiles, plus a pledge to continue negotiations for "substantial reductions" in nuclear arsenals.

Restraints on cruise missiles and the heaviest Soviet land-launched missiles are at the center of this continuing dispute.

The negotiations now taking place in Washington were originally scheduled for Sept. 7-9 in Vienna. They were postponed not only because of American-Soviet disagreement, but also because of discord inside the Carter administration, including dispute over range limitations on air-launched cruise missiles.

President Carter's decision to abandon development of the B-1 bomber has brought intensified pressure from the Pentagon for U.S. insistence on greater range for American air-launched cruise missiles.