The Soviet Union has started to use the Vietnamese port of Cam Ranh Bay, on which the United States spent $2 billion during the Indochina war, for its submarines, government sources revealed yesterday.

American spy satellites recently photographed at least one Soviet Foxtrot submarine sailing in and out of Cam Ranh, sources said. U.S. Navy leaders believe this additional Soviet presence in Vietnam is highly significant.

Not only is Vietnam shaping up as a launching site for Soviet reconnaissance missions by TU95 Bear bombers and surface ships, in the view of Navy intelligence, but it also is becoming a stopover point for submarines on their way to and from the Indian Ocean.

The Foxtrot, a diesel attack sub designed to sink ships or other subs with its 22 torpedoes as distinguished from firing missiles against land targets, is not in itself considered a major threat. It is the widening Soviet use of Vietnam as a military base that worries U.S. intelligence.

Vietnam watchers at the Pentagon are reporting a heavy step-up of Soviet shipments of goods into Vietnam this year, much heavier than in 1978. Some specialists theorize that Ho Chi Minh's successors running Vietnam are trading basing rights ot the Soviet for manufactured goods.

Thanks to U.S. efforts during the Indochina war, Vietnam has superb military facilities for a superpower fields at Da Nang and Ho Chi Minh City, formerly Saigon, plus the deep water port of Cam Ranh Bay, are ready-made for long-range planes, ships and submarines.

The United States spent$2 billion improving Cam Ranh Bay during the Vietnam war, according to an Army heriting everything there, from piers to barracks and mess halls for ship and sub crews.

Although American military leaders are fretting about the Soviets' growing use of Vietnam, the Japanese government so far has expressed the greatest alarm.

Japanese Prime Minister Masayoshi Ohira has indicated that whether his country will continue giving economic aid to Hanoi depends on whether Hanoi allows the Soviets to establish permanent military bases in Vietnam.

The sea is Japan's lifeline, with almost all of its oil coming by ship from the Middle East. The fresh evidence that the Soviets are using Cam Ranh Bay for submarines, which in wartime would threaten Japan's lifeline, is sure to heighten Japaneses fears about the growing Russian military presence in Asia.

Sources said it is too early to predict how Soviet submarines will use Cam Ranh Bay. One theory is that it will become a stopover point for changing crews and resupplying the submarines in trips between Vladivostok, a major port for the Soviet Pacific fleet, and the Indian Ocean.

The Indian Ocean is growing in strategic significance because it is the access route to the oil-rich Persian Gulf. The U.S. and Soviet navies have been rotating task forces in and out of the Indian Ocean lately.

One likely mission of the Foxtrot submarine, once it reaches the Indian Ocean, is to shadow the American aircraft carrier sailing with the U.S. naval task force.

U.S. Navy strategists have long regarded Vladivostok, on the Sea of Japan, as a port they could bottle up easily if war came. But Cam Ranh Bay as a major Soviet military port would raise a whole new set of problems, according to Navy leaders.

Last month, The Japanese government went public with its suspicious that Soviet TU95D Bear Bombers equipped for reconnaissance were flying in and out of Vietnam. Two Bears were spotted on the ground at Da Nang airfield, a major base for U.S. warplanes during the Vietnam war.

Earlier, U.S. intelligence had reported that the Soviets were increasing the number of surface ships stationed off Vietnam and using Cam Ranh Bay as a port.

The enlarged Soviet presence in Vietnam may be designed to scare off China or at least signal the world that Moscow intends to stand by Hanoi in its conflicts with Peking.

The United States has no legal right to object to Soviet use of Vietnamese territory, military leaders concede, but the presence is infuriating to some U.S. OFFICERS IN LIGHT OF ALL THE AMERICAN BLOOD AND TREASURE SPENT THERE DURING THE VIETNAM WAR. CAPTION: Picture 1, At least one diesel-powered submarine of the Soviets' Foxtrot class, such as the one shown above, has been photographed sailing in and out of Cam Ranh Bay. U.S. Navy Photo; Map, Prospective route of Soviet submarines. By Richard Furno - The Washington Post.; Picture 2, Vietnam is also shaping up as a launching site for reconnaissance missions flown by Soviet TU95 Bear Bombers, such as the one shown above. U.S. Navy Photo