Secretary of State Cyrus R. Vance today will give the Senate Foreign Relations Committee an explanation of the new U.S.-Soviet nuclear arms control compromise, before facing a more critical Senate audience Friday.

The Foreign Relations Committee, headed by Sen. John J. Sparkman (D-Ala.), is basically "dovish" and friendly to American-Soviet compromises.The sterner test for the Carter administration's nuclear negotiations is the more "hawkish" Senate Armed Services Subcommittee on Arms Control, headed by Sen. Henry M. Jackson (D-Wash.).

Both committees asked for Vance's testimony in closed hearings this week, and the scheduling that was worked cut brings Vance before the milder forum first. It is the Jackson subcommittee, however, which starts hearings Friday, that will determine how much criticism the administration will encounter as it proceeds with the U.S.-Soviet strategic arms limitation talks (SALT) in Geneva.

Administration strategists have anticipated for months that they were bound to encounter strong domestic opposition to any SALT compromise with the Soviet Union. But there is uncertainty at this stage whether a major confrontation is near, or whether critics will hold their fire until the nuclear negotiations are further advanced.

What administration officials hoped to avoid was a new controversy about foreign policy breaking out on top of the running dispute over ratification of the Panama Canal treaties. The pace of negotiations, however, has now thrust into the same time frame the administration's diplomacy on Panama, on SALT and on the Middle East, on which Vance also will be reporting to the Senate Foreign Relations Committee.

President Carter, who told the United Nations last week that Washington and Moscow "are within sight" of a new nuclear accord, yesterday appeared to be cutting back on optimism.

Yesterday Carter said that "the results so far are encouraging," but he went on to say, "We don't know yet what the Soviet attitude is on very important remaining differences."

He said, "We have nothing to indicate that there have "been irresolvable differences." And yet Carter also said, "We don't know how much progress will be made in the future."

This ambivalent attitude suggests that the administration hopes to head off potential critics of its negotiating strategy with the contention that judgments on the outcome are premature while bargaining continues.

At the same time, the administration is anxious to convince Congress that enough progress is being made in the SALT talks to justify pursuing them without a disruptive domestic debate.