WILL THE United States soon be vulnerable to a sudden thermonuclear attack by the Soviet Union that could wipe our America's land-based missile force in a single blow? Yes, reply most of the experts - even the president of the United States. The 1980s will be perilous for America because of this "vulnerability," according to Henry A. Kissinger and many others.

A seemingly simple mathematical fact leads to this gloomy forecast. By the early 1980s the Soviets will have enough accurate warheads on their land-based rockets to target two bombs on every American missile silo while still holding in reserve most of their offensive forces. Theoretically, this would give the Soviets high confidence of "killing" 90 percent of the American silos.

In the past, America has relied on its "triad" of strategic forces - the combination of land-based missiles, submarine-based missiles and bombers - to insure survival of an adequate strategic force. If two or even one of those systems survived, that would be adequate to destroy the Soviet Union, according to the theory.

In a time of steadily increasing Soviet forces, however, the original "triad" theory has fallen victim to a new argument. Simply stated, the new theory is that even the appearance of a Soviet strategic advantage over one of the three elements of the "triad" would be dangerous. "Perceptions" about vulnerability are just as important as reality, according to this theory: the Soviets or other nations could misinterpret this perception as a genuine sign of American weakness. This is a theory devised and promoted entirely by Americans.

However, the most important question about vulnerability has not been asked: How would the Russians see it?The Soviet military literature contains no analysis of a possible Soviet first strike against U.S. land missiles. They how might Soviet military planners exploit "vulnerability" five years from now, when their theoretical advantage will be greatest?