Vital national interests

President Reagan is attempting to bolster a regime in El Salvador on the grounds that its overthrow by the rebels in the back country would endanger ''vital national interests'' of the United States.

President Reagan is reported to have approved a clandestine ''destabilization'' program to try to bring down the present Sandinista government in Nicaragua. The reason given for the program is to protect ''vital national interests.''

The question is whether giving guns to the Duarte regime in El Salvador and encouragement to refugee Nicaraguans is the best way to sustain what are truly vital US interests in Central America and in the Caribbean basin.

Have we thought through sufficiently just what those vital interests are?

There is one vital interest on which most Americans can undoubtedly agree. It is vital that no potentially unfriendly country be allowed to plant major strategic weapons in the area.

That was the issue in the Cuban missile crisis. That crisis was resolved on the promise of the Soviets to withdraw their intermediate range nuclear missiles from Cuba and to refrain from attempting to base such lethal weapons there again.

It has been axiomatic ever since that the US will allow Cuba to have whatever system of government it likes, provided that it does not become a base for potentially hostile weapons capable of reaching and damaging the US or its interests in other places in the area. This has been interpreted to mean no Soviet submarines or other warships armed with strategic weapons using Cuban harbors.

Are there other vital interests?

Yes. Washington could not tolerate an unfriendly or potentially hostile government gaining control of the Panama Canal.

Beyond that is it vital that the governments in Central America forswear ''Marxism'' and declare themselves to be true believers in ''capitalism''?

Certainly not. Naturally, Washington would prefer its southern neighbors to practice marketplace economics. And there is always an uneasy feeling that if a country goes ''Marxist'' it will tend to move into Moscow's power orbit. But the fact that Cuba is both Marxist and in the Soviet power orbit has been a mixed disadvantage to the US. The condition of unfriendliness which exists between Cuba and Washington has involved Cuba sending soldiers overseas in support of Soviet operations. It also involved Mr. Castro exporting hundreds of his worst criminals to the US.

On the other hand, unfriendliness between Havana and Washington has relieved Washington of subsidizing the economy of Cuba. That subsidy, which now runs into several million dollars a day, has become a steady drain on the Soviet economy. Resumption of friendly US relations with Cuba would relieve Moscow of that drain and probably lead to a resumption of a US subsidy.

Right now there are tentative and exploratory negotiations going on toward a possible resolution of the Central American ''crisis'' which has been making front-page news for several weeks. The Mexicans, who have been acting as an intermediary, seem to think that there is a chance of success. Their optimism will be justified depending on what Mr. Reagan in Washington regards as ''vital.''

There is no reason why the Cuban formula would not be acceptable to Nicaragua. There are Marxists among the ruling junta in Managua and the trend is toward more Marxism in the economy rather than less. Obviously, the regime in Nicaragua is not going to renounce the right to use Marxism in its economic programs if it thinks it will work better than capitalism.

But there is no reason to think that the members of the junta in Nicaragua have any desire to have their country become a base for Soviet strategic weapons. The sticking point could come over whether they have a right to ferry guns across the bay to left-wing rebels in El Salvador.

The obvious way out would be a mutual nonintervention agreement, the US to refrain from aiding the official government in El Salvador if the Nicaraguans will refrain from aiding the rebels. Such an agreement could be monitored by the Mexicans. But is Washington willing to leave the internal political solution in El Salvador to the Salvadorans?