Vietnam is showing signs of anxiety about a possible rapprochement between its arch-enemy, China, and its strongest supporter, the Soviet Union.

Yet it remains unclear how the efforts by Moscow and Peking to improve their strained relations will affect the situation in Cambodia, where Vietnamese troops have been battling Cambodian guerrillas since an invasion nearly four years ago. The Chinese have demanded a Vietnamese withdrawal from Cambodia as one of three conditions for improving relations with Moscow.

Some Western diplomats believe that the Vietnamese may launch a bigger offensive than usual against resistance groups near the Thai border during the current dry season to try to upset the Sino-Soviet rapprochement, among other reasons.

While the Sino-Soviet feelers have been showing promise, there has been no sign of any positive Chinese response to recent ostensible Vietnamese overtures on improving relations.

In August, the Vietnamese offered China a six-week cease-fire along their common border, which Peking promptly and emphatically rejected. Hanoi is not known to have offered anything new in Cambodia.

China late last month accused Vietnam of trying to aggravate relations by claiming the Xisha and Nansha island groups, also known as the Paracels and Spratleys, in the Gulf of Tonkin. China described as "illegal and null and void" Hanoi's Nov. 12 statement claiming the islands and said Vietnam "must bear full responsibility for all serious consequences that may arise."

The Vietnamese Communist Party newspaper Nhan Dan meanwhile accused Peking of rejecting Hanoi's peace initiatives in Cambodia and escalating weapons deliveries to the Khmer Rouge guerrillas. The paper also denied that Vietnamese forces were preparing for an offensive against the communist Khmer Rouge and two other Cambodian resistance factions during the current November-to-May dry season.

According to the head of Thailand's National Security Council, Squadron Leader Prasong Soonsiri, the Vietnamese clearly are worried about the Sino-Soviet rapprochement. He cited the recent visits to Moscow of high-ranking Vietnamese officials, adding, "if you're not worried, you stay at home."

In October, Truong Chinh, Vietnam's president and number two man in the Communist Party Politburo, went to Moscow at the same time that Soviet Deputy Foreign Minister Leonid Ilyichev was in Peking for exploratory talks with the Chinese. The Vietnamese visit was announced shortly after the Soviets and Chinese said they planned to meet in Peking.

A communique issued after Truong's talks with the late Soviet president Leonid Brezhnev said the discussions were largely devoted to Sino-Soviet and Sino-Vietnamese relations and that the two leaders agreed that any normalization between Moscow and Peking would not come at the expense of "third countries." The communique's tone toward Peking was unusually mild, and China was not criticized by name.

On the same day that the Vietnamese party paper, Nhan Dan, published the communique, however, it also printed a harsh denunciation of China as "the principal danger to the three Indochinese countries." It said the three, Vietnam, Laos and Cambodia, were "determined to defeat all maneuvers by the reactionaries in the Peking government."

When a Vietnamese delegation led by Truong Chinh returned to Moscow for Brezhnev's funeral last month, its reception seemed cool for such a close ally. The new party chief, Yuri Andropov, did not meet with Truong Chinh, and a deputy foreign minister received Vietnamese Foreign Minister Nguyen Co Thach and Cambodian Foreign Minister Hun Sen.

Indochina watchers here noted that Hun Sen was kept out of one ceremony because of the presence of the then Chinese foreign minister, Huang Hua.

"I think the Vietnamese are looking over their shoulder at the Russians," said a senior Western diplomat who has served in both Moscow and Peking. "They're worried about the Sino-Soviet relationship, but there's no sign it's having any impact on their activities in Indochina."

The Vietnamese are believed to retain bitter memories of 1972 when the United States began to improve relations with the Soviet Union and China.

"The results from Vietnam's standpoint were damaging," the diplomat said. The Americans mined Haiphong harbor, but a U.S.-Soviet summit meeting went ahead anyway.

According to diplomatic sources, a possible major ramification of Sino-Soviet rapprochement could be a reduction of Soviet Bloc aid to Vietnam, estimated at more than $2 billion a year. The aid is considered vital to the Vietnamese war effort in Cambodia and a source of irritation between Peking and Moscow.

This was emphasized last Wednesday when news reports from Peking quoted an unidentified senior Chinese official as saying that Soviet support for Vietnam's military intervention in Cambodia was the most important obstacle to improved Sino-Soviet relations. The official also said no date had yet been set for the next round of Sino-Soviet talks but that they would not resume until next year.

According to a diplomat who recently returned from a trip to Vietnam, Eastern European countries already are planning to cut back aid to Vietnam by about 20 percent. He noted that during Truong Chinh's visit to Moscow in October, Soviet leaders openly criticized Hanoi for inefficient use of foreign aid.

However, the Soviets are seen here as having little flexibility as far as China's demand for Vietnamese withdrawal from Cambodia is concerned. Any outright Soviet pressure on Vietnam to pull out its troops would almost certainly meet a stiff reaction, and the Soviets would stand to lose naval and air base facilities at Cam Ranh Bay and Danang, diplomats said. Vietnam allowed the Soviets to use the bases after China's incursion across Vietnam's northern border in 1979.

Another Chinese demand, a reduction of Soviet forces along the Chinese border, is easier for Moscow to meet, but may also provoke Vietnamese anxieties, according to diplomats here.

"Anything that eases pressure on China's northern border is a problem for Vietnam," a diplomat said, since it would allow the Chinese Army to focus more attention on the Vietnamese border.

The problem was underscored for Hanoi last month when Chinese Premier Zhao Ziyang told visiting Thai Prime Minister Prem Tinsulanond that China would give the Thais "complete support" if the Vietnamese invaded Thailand.

To wipe out the Cambodian resistance groups along the border, military analysts here say, Vietnamese forces must cross into Thailand in strength and surround them, an act that might provoke the threatened Chinese reaction.