Bangkok — Vietnam's diplomatic efforts to mend relations with its neighbors are proceeding slowly through polite discussions and trusted intermediaries. The polite discussions were the stuff of Vietnamese Foreign Minister Nguyen Co Thach's recently concluded trips to the Philippines and Thailand. But the comments Mr. Thach made about China during his Bangkok visit were especially interesting to Indochina watchers here.
''We are trying our best to have contacts and negotiations with China,'' he told journalists, adding Hanoi had already asked some ''friends'' to act as intermediaries between Vietnam and China.
He did not give the identity of the friends, but France seems the most likely candidate. It retains fairly good relations with its former Indochinese colonies , and French Foreign Minister Claude Cheysson is an old Indochina hand.
Low-level contacts between Chinese and Vietnamese officials may in fact have already happened. This might explain the unusually conciliatory tone of Mr. Thach's references to China.
''China and Vietnam were born to coexist,'' he told journalists. ''They have already coexisted for 4,000 years.''
Until very recently the Vietnamese have been more likely to refer to 4,000 years of Chinese ''aggression.''
Part of the new-found conciliation may, however, have been based on a misapprehension: Mr. Thach and his advisers were under the impression that China's premier earlier last week had proposed talks between their two countries. In fact, Zhao Ziyang had simply repeated his country's earlier terms for negotiations - terms that Hanoi had already rejected - during his report to the sixth National People's Congress.
As for Vietnam's relations with Thailand, meetings between Foreign Minister Thach and his Thai counterpart, Air Chief Marshal Siddhi Savetsila, seem to have slipped into a predictable pattern. First, both men usually surprise observers by the warmth with which they greet each other. This sparks off speculation that a solution to Vietnam's occupation of Kampuchea is imminent. But then, when the clouds of euphoria clear, there is nothing solid left behind.
This happened when Mr. Thach came here a year ago, and seems to have happened again last week.
Last Thursday's meeting was ''friendly and successful,'' Mr. Thach told reporters. He seems to have suggested that the Kampuchea problem can, in Hanoi's view, be divided into two threats:
The first one is to Kampuchea, from anti-Vietnamese guerrillas along the Thai-Khmer border and the supply line that channels weapons and other materiel from China and Singapore to the Khmer Rouge and its two smaller noncommunist allies. Thailand denies that its territory is used for supply purposes. The second, greater threat, in Mr. Thach's view, comes from China.
But the security of both Kampuchea and Thailand, he told journalists in Bangkok, could be ''partially guaranteed'' before the conclusion of an overall settlement of Kampuchea. This idea bears a strong resemblance to a proposal the Indochinese nations have been pushing for some time: a demilitarized zone along both sides of the Thai-Kampuchean border. The zone would be closed to Vietnamese troops on the Khmer side, and to all but Thai soldiers on the other side. The Association of Southeast Asian Nations (ASEAN), consisting of Thailand, Singapore, Indonesia, Malaysia, and the Philippines, so far has shown no interest in the idea.
Mr. Thach's other suggestion also seems to be a variation on an old theme. With the studied politeness of someone debunking an old friend's pet theory, he suggested that the ASEAN approach to the Kampuchea problem was a failure. He has done this many times before, but usually with less diplomacy. Instead of pursuing a solution to the Kampuchea question - which might in the future aggravate the ''40 years of mistrust and confrontation'' which he says have bedeviled the countries of ASEAN and Indochina - Mr. Thach suggested they should all ''set aside'' the question. They should look for a ''framework for peaceful coexistence,'' considering the general questions of regional security first, and returning to Kampuchea later. ASEAN has rejected the idea in the past, and seems unlikely to accept it this time around.
One tangible point to emerge from the talks was that Thai Foreign Minister Siddhi apparently agreed to visit Hanoi at some unspecified future date.