It takes a strong and determined man to be an optimist in this capital where gloom and pessimism seem to discourage even the first crocuses of spring. It also takes a man with wide knowledge and calm confidence. That man is Secretary of State Cyrus Vance.

Vance believes a SALT II agreement will be reached with the Soviet Union perhaps as early as midyear. Notable progress has been made in negotiations with Moscow since Foreign Minister Andrei Gromyko attended the U.N. General Assembly and came to Washington for two days of meetings with President Carter and Vance last September. Vance had several sessions with Gromyko in New York.

The secretary's conviction differs from the view of the president's national security adviser, Zbigniew Brzezinski. Brzezinski holds that the American side has made all the concessions that are possible and movement must come from Moscow if there is to be movement.

Vance readily acknowledges that some sticky points remain still under negotiation. They are not insuperable obstacles, although it may develop that they can be resolved only at a summit meeting between Leonid Brezhnev and President Carter. That could take place as early as June with Brezhnev in Washington, since an American president, Gerald Ford, went to Vladivostok in the Soviet Union for a meeting with the Soviet leader.

Vance opposes linking the arms negotiations with Russian intervention in the Horn of Africa. Here, too, he differs with Brzezinski, who, while denying "linkage," seemed in a recent statement to be tying an arms-limitation agreement to the use of Russian and Cuban troops in the war between Ethiopia and Somalia.

There is a chance, a good chance in the view of the optimist, that war may be concluding. President Carter has offered the Somalis military and economic aid if they withdraw from the Ogaden region. Then the question is whether Moscow will order 11,000 to 15,000 forces to pull out of Ethiopia.

The possible consequences of the Soviet intervention in public opinion in relation to the ratification of a treaty are apparent enough. But Vance puts an arms-control treaty with iron-clad provisions for verification above everything else as one more step toward slowing the nuclear arms race. While it may not go before the present session of Congress, it would be put before the Senate in 1979; less chancy, since that is not an election year.

The sticking points are formidable enough. One most frequently dicussed is the range of the American low-flying cruise missle. Progress has been made, however, in relating its range to its life as an active self-guided missile before its extinction. That is equated with the range of the Soviet Backfire bomber and whether its capability extends to Western Europe and beyond.

Failure to achieve a new arms-control agreement would mean that new weapons all but ready to come on the line would be deployed and any future cutback of weapons would be far more difficult, if not impossible. The Soviets have five new missiles, some of which have already been tested. Without restraints imposed by a new agreement, they would become part of the Soviet arsenal.

Two new Soviet submarines are ready for sea trials. Contrary to published reports, they have not been added to the Soviet submarine fleet, and therefore that fleet does not exceed in numbers the SALT I limits on offensive weapons. Without a new agreement they would become part of the offensive force.

So many elements make this a crucial moments as to which path the superpowers take; whether an accelerated race toward a nuclear holocaust, or a pause that can mean scaling back the mass of weapons sufficient today to destroy civilization two or three times over. One element is Brezhnev's health. He is a known quantity, as against an unknown who would succeed him.

After the Vladivostok encounter, official sources here believed the Soviet leader was suffering from leukemia, a form of cancer of the blood nearly always fatal. That was based in part on the puffy appearance of his face, which resembled that of France's President Georges Pompidou, who had been taking cortisone and who later died of what was reported to have been a blood disease.

Intensive intelligence efforts have thus far failed to confirm the leukemia report, although it is accepted by some high official sources. Brezhnev has other known ailments of a serious nature. He wears a pacer, which has recently been readjusted, in connection with a heart condition. A longtime hard smoker, he suffers from emphysema.

While Vance weighs the obstacles in the way of an agreement, he is undaunted by them. He is dismayed at times by the tactics used on Capitol Hill, as when a Senate staffer leaked the testimony he had given in executive session to try to discredit his aims. But with his low-profile, unemotional style, he plows ahead toward a goal he believes is imperative for the survival of humanity.