United Nations, N.Y. — ''The Southeast Asian glacier is beginning to melt,'' says a Western diplomat experienced in Far Eastern affairs. ''The evolution is slow, but the way things are shaping up,'' he continues, ''ASEAN countries led by Indonesia might in a few years patch things up with the Indochinese countries.''
Vietnam, which has occupied Kampuchea (Cambo-dia) since January 1979, is not about to withdraw its troops in the near future. But Hanoi has made it clear that it will withdraw its troops from Kampuchea if there is an end to the ''Chinese threat,'' if the use of anti-Vietnamese Khmer Rouge forces is stopped, and if Thailand's territory is no longer used as a springboard for Khmer Rouge incursions into Kampuchea.
At the same time, it appears that relations between Vietnam and Indonesia - the largest, most populous, and wealthiest country in the Association of Southeast Asian Nations (ASEAN) - have improved.
Last month, Indonesia's foreign minister, Mochtar Kusumaatmadja, publicly expressed his disappointment at Vietnam's refusal to withdraw its troops. But the Indonesian commander of the armed forces, Gen. L. B. (Benny) Murdani, also said that ''Vietnam was not a threat to Southeast Asia.''
In a visit to the Vietnamese-Chinese border last month, General Murdani and 14 other Indonesian generals behaved as if ''Vietnam and Indonesia were allies, eager to contain Chinese expansionism,'' an Indonesian military source said privately.
General Murdani was quoted as saying that ''our two countries are close friends and will become even closer.''
Such statements have added weight since, according to highly placed Southeast Asian sources, Indonesia's President Suharto is closer to his military than to his civilian establishment.
An Asian diplomat says, however, that ''the Javanese way of doing things is not through confrontation but through walking in circles toward the objective.
''Indonesia feels that it deserves to be the leader of ASEAN, because of its geopolitical weight and it now begins to make its weight felt. Since the attempt by the pro-Chinese Communist Party in Indonesia to take over in 1965, the Indonesians consider China, not Vietnam, to be their main foe.
''In fact,'' the official continued, ''while Indonesia happens to be inside China's vital interest perimeter, Thailand is outside Indonesia's perimeter, which includes Malaysia and the Philippines.
''What we are going to watch is Indonesia asserting itself more and more inside ASEAN, winning the Philippines and Malaysia over to its views, and leading ASEAN away from its tough anti-Vietnamese stance.''
Even Thailand may have to talk with the Vietnamese.
''Thailand already sees the sign on the wall,'' an ASEAN diplomat said, ''and may soon begin to discreetly seek a bilateral accommodation with Vietnam on its own.
''This means that in the end, Thailand would settle for less than it had hoped for. Cambodia will not become a neutralized buffer state between Thailand and Vietnam. Instead, Vietnam would give Thailand iron-clad security guarantees, '' the diplomat predicted.
This scenario would leave Kampuchea and Laos under Hanoi's influence if not under Vietnamese occupation.
Significantly, Indonesia has told Vietnam that it does not want Australia to play the leading role in bridging the gap between ASEAN countries and Vietnam. Indonesia wants to be the main player in this issue.
Although neither France nor Japan is exerting pressure on ASEAN countries, they have both been discreetly telling them for some time that it may be counterproductive to push Vietnam further into the Soviet corner ''out of desperation.''
Another close observer, however, points out that ASEAN has gained much by taking a unified stand on Kampuchea. No ASEAN country, this source says, is willing to break this unity. Moreover, Vietnam is worried that a diplomatic thaw between the Soviets and the Chinese could work against it.
''The Vietnamese know from experience that the Soviets could reduce their economic and military aid to them if they felt it were in their own interests,'' an Asian ambassador here observed. ''Thus they are increasing their efforts to drive a wedge between the ASEAN countries and China.
''Aside from the Indonesians, other ASEAN officials are beginning to fear that China might ultimately turn Southeast Asian countries into satellites, just as the Soviets rule Eastern Europe. . . .''