The passage of Secretary of State Vance from darkest Africa to darkest Russia invites a shot at the silliest notion affecting U.S. foreign policy today. I mean the myth that by acting tough on arms control in Moscow, the United States can stop the Russians from being bad boy in Africa.

In fact, arms control and Africa are not candidates for linkage. Not only is one important to the United States and the other not, but morever a stance of confrontation on either would be adverse to American interests on both.

Arms control is perhaps the one subject that truly engages American national security. It, and it almost alone, bears directly on the prospects for nuclear war and national devastation.

A series of treaties with the Soviet Union have already worked to limit that danger. By the ABM treaty of 1972, both sides renounced the building of defenses against nuclear missiles. Each, accordingly, made its population hostage to the missiles of the other - thus loading the odds heavily against any deliberate nuclear attack.

Treaties limiting offensive weapons have been less successful. Each side continues to maintain and improve a vast arsenal of nuclear arms. But limitations on numbers of strategic weapons were accepted in the interim agreement of 1972 (SALT 1), which expired last October.

The follow-on agreement now being negotiated (SALT 11) brings down limits on the total number of launchers permitted to each side, restricts the number that can carry multiple warheads, and sets bounds on the testing and development of new weapons.

Apart from tightening limits, moreover , SALT II imparts momentum to a process that was starting to run down. It keeps in being a valuable, jointly accepted system of monitoring and inspection. It engages the Russians in the arm-control game at a time of leadership change when the successor to General Secretary Leonid Brezhnev may be less keen on agreement. It thus sets the stage for new negotiations on a third arm-control agreement.

The United States for all those reasons has a strong interest in having another arms-control accord rather than in not having one. Threatening not to sign, accordingly, is a naked bluff. It is a bluff sure to be called when set against the picture of Africa.

The Russians, to be sure, have been expanding their influence here in big way. They and their Cuban allies implanted themselves in Angola by aiding the faction that eventually took over the country when Portugal withdrew.

They fueled a war in the Horn of Arica by equipping Somalia with modern weapons for a fight against the Ethiopia of Haile Selassie. When a military junta ousted the emperor, the Russians switched sides. They have now intervened massively, again with Cuba, to repel a Somali invasion of Ethiopia's Ogaden province. They are about to help the Ethiopians take on a secessionist movement in the province of Eritrea. so they stand aces high with the revolutionary regime in Addis Ababa.

In Rhodesia there seems to be shaping up a civil war pitting the moderate blacks, who have made a settlement with the white regime of Prime Minister Ian Smith, against the more radical black Rhodesians, backed by the neighboring black African states. The Russians and Cubans may well intervene on the radical side. They would thus align themselves with blacks against whites, a winning position in Southern Africa.

Blocking Soviet expansion in Africa is the U.S. interest, not because the continent is so important but because the Russian appetite tends to grow with eating. However, the United States has no good countervailing power immediately available in the continent.

Washington is surely not going to intervene directly - especially not against whites. It's anti-communist Arab allies - Egypt, Saudi Arabia and the Sudan - have just proved their ineffectiveness in backing Somalia against Ethiopia.

Finally, there is the tactic most dear to the Carter administration - the tactic of backing Nigeria, the biggest African state, and using Ambassador Andrew Young to spout idealistic views on human rights and self-determination. That may have a great future.

But it does not commmand significant Amercian support. Neither does it generate the kind of excitement among Africans that the radical approach pushed by the Russians and Cubans engenders. So the American interest in Africa is to play for time, and hope that the Russians will - as they have so often in the past - spoil their own game. Any immediate confrontation in Africa is a loser for the United States, and coupling it with a tough stance on arms control is a double loser.