Vietnam appears to be showing increased interest in better relations with Washington, report foreign diplomats in Hanoi. One subdued but friendly signal to the US came at last month's summit of officials from Vietnam, Kampuchea, and Laos. The final document of the Hanoi-arranged meeting notes the ''great importance'' that the three Indochinese countries attach to friendship with the American people. It goes on to express the desire for normalized relations with the United States.
In Hanoi, a source close to the Vietnamese cites one stumbling block: ''It would be nice to see the US moving slightly away from its present attitude of paralleling Chinese policy in Southeast Asia.''
He adds: ''If the Americans are really interested in limiting the influence of a certain other big power in the region, perhaps they should start talking to us.''
That other big power, the Soviet Union, up to now has been a close ally of Vietnam. But Hanoi has cast a wary eye on the recent talks between the Soviets and Hanoi's sworn enemy, the Chinese. The second round of those talks opened this week in Moscow.
If Vietnam is worried about its Soviet ties, the feeling shows in its tentative efforts to diversify its foreign relations.
These had their beginning here last week when Vietnamese Foreign Minister Nguyen Co Thach announced that his government and its Khmer allies had decided to withdraw a second batch of Vietnamese troops from Kampuchea this year. In the future, Mr. Thach said, there would be regular partial withdrawals of Vietnamese troops each year if the security situation allowed.
The withdrawal offer seemed to be a partial response to the detailed plan that the Chinese had presented to their Soviet interlocutors during the first round of Sino-Soviet talks in Peking last October. At that time the Chinese said that if Vietnam would agree in principle to pull its troops back unconditionally from Kampuchea, normalization talks between Hanoi and Peking could begin immediately.
The trouble is that the Vietnamese withdrawal is conditional: Hanoi will not bring all its men home until Peking agrees to end all aid for the anti-Vietnamese Coalition Government of Democratic Kampuchea.
And Hanoi clearly considers that China's other main demand - making Kampuchea neutral - is totally out of the question.
Perhaps because of this China dismissed the idea of troop withdrawals as a ''hoax,'' but their call to Vietnam to make ''joint efforts'' with China to overcome obstacles to better relations suggests that Peking still sees some chance of improvement.
Rejection or not, the Vietnamese apparently expect their Soviet allies to discuss the new proposals with the Chinese in the course of the present talks.
The Soviets in fact have every reason to be pleased with the Vietnamese initiative. Well-informed sources here say that for the last six or seven months - in other words since the first round of Sino-Soviet talks - Moscow has been urging its allies to make just this sort of gesture to Peking.
The sources say that Moscow would like to see the army of Heng Samrin's People's Republic of Kampuchea (PRK) take over most of the responsibility for security inside Kampuchea by 1985. The PRK army is currently said by noncommunist sources to number about 48,000 men, and to be receiving Soviet aid of around $25 million a year.
The Soviets are, however, reportedly using persuasion rather than pressure. They point out that 1985 will be an important year for Vietnam - the 40th anniversary of the August revolution against the French, the 10th anniversary of the capture of Ho Chi Minh City (Saigon) from the Americans. A number of major Soviet aid projects are due to be inaugurated that year. ''So the Soviets are saying,'' one source said, ''why not round out the year nicely with a declaration that the situation in Kampuchea has now been normalized.''
Vietnam is apparently dubious about a rigid timetable, but other reports here suggest it is already giving the PRK troops a larger role to play in military operations.
Informed sources here assert that Khmer troops are beginning to replace Vietnamese troops along most of the Thai-Khmer border, and that in many places Vietnamese soldiers are moving 10 to 15 miles away from the frontier. The Vietnamese do, however, keep advisers in Khmer units down to the platoon level.
The Vietnamese do not seem too concerned by China's negative reaction.
''They're like that,'' one official remarked. ''They're tough and stubborn - and slow - till the last minute. Then when they decide it's worth their while, they'll shift.''
But even if this does happen, Vietnamese officials note, there will be no overnight reconciliation. Vietnam and China have a number of disputes other than Kampuchea - the ownership of two island groups in the South China Sea, for example, and delineation of the potentially oil-rich Gulf of Tonkin.
''If you want precedents for subjects like that,'' said a Vietnamese official , ''look at Peking's border talks with India. They've been going on for 20-odd years without an agreement, and the Chinese (and Russians) are just beginning to think about discussing border problems that date back to the 19th century.''