Five or six months ago, most of the Indochina-watchers in Bangkok were predicting a Vietnamese dry-season offensive aimed at wiping out the Khmer Rouge guerrillas in Cambodia.

Today, with the rainy season only a few weeks away, the dry-season offensive has still not materialized, and it appears that the Communist rulers of Vietnam have been telling the truth. They have insisted all along that there was not going to be any dry-season offensive.

There could be one yet. Or there could be a wet-season offensive. As anyone who served in Vietnam can testify, the war did not stop when the rains started.

But it would be hard to find anybody around here who thinks there will be an offensive, wet or dry. The military experts who were positive that the Vietnamese would move in to finish off the Khmer Rouge forces of Pol Pot have lapsed into embarrassed silence.

The remnants of Pol Pot's peasant army, estimated to number 20,000 to 30,000, are still fighting. Their leaders are claiming that Vietnam's offensive began but that the Khmer Rouge beat back the attackers.

It had been predicted that the Vietnamese would begin their offensive with the arrival of the dry season in November. When it failed to come off, the experts said the Vietnamese would surely begin their big attack on Jan. 7, the first anniversary of their capture of Phnom Penh.

When again nothing happened, the experts pointed to Tet, the Vietnamese lunar new year, in February. The Vietnamese, it was argued, are fond of anniversaries.

Nothing happened, and the experts stopped talking about a dry-season offensive.

The Vietnamese have dismissed Pol Pot's forces all along as bandits with nothing more than nuisance capability. This is something of an exaggeration, but it does appear to be true that the Vietnamese and their satellite government in Phnom Penh effectively control the cities and rice-growing central plain of Cambodia.

With an estimated 200,000 troops in Cambodia, the Vietnamese have the resources for a major offensive if they want to mount one. But Pol Pot is not a political threat to them. As much as Cambodians hate the Vietnamese, they say they hate Pol Pot even more, for under his rule an estimated 2 million Cambodians died of torture and starvation.

Another reason for not launching an offensive to wipe out the Pol Pot forces is that it would mean bringing the war into Thailand. Khmer Rouge forces in western Cambodia often cross into Thailand when the going gets too hot for them.

Still, despite considerable provocation from the Thais, who openly support the Khmer Rouge, the Vietnamese have tried to confine the war to Cambodia. This is not out of solicitude for the Thais but out of fear of the consequences.

Any sizeable Vietnamese military thrust into Thailand could provoke a Chinese thrust into Laos, a Vietnamese satellite, or another Chinese attack on Vietnam.

Accordinging to observers here, it might even provoke a military reponse from the United States. As these observers see it, U.S. failure to respond to a Vietnamese invasion of Thailand would destroy what credibility the United States has left in this part of the world.

They think that Vietnam cannot afford to risk the wrath of both China and the United States. Moreover, they think that any move against Thailand would wreck any prospect of Hanoi reaching an accommodation with its Southeast Asian neighbors.

Still another reason for confining the war to Cambodia is the Soviet Union, Hanoi's chief supporter. The Russians, bogged down as they are in Afghanistan, would presumably not wish to underwrite any military adventures in Southeast Asia.

Meanswhile, there is evidence that the Vietnamese are fed up with their Cambodian adventure.

A Western diplomat who recently toured much of Cambodia said that Vietnamese disenchantment is widespread. He talked with a Vietnamses soldier who said he was sent to fight in South Vietnam in 1973 and has not been home since.