Food and gasoline are running short in Laos, Vietnam's client state, three weeks after Thailand closed its 900-mile border with the landlocked country. The blockade appears to be part of Thailand's larger dispute with Vietnam.

The frontier was closed, Thai officials say, in response to the June death of a Thai Marine when Laotian forces fired on a Thai patrol boat in the Mekong River, which forms part of the border between the two countries.

Many diplomats here believe, however, that the Thai move was intended as a signal of displeasure to Laos' patron, Vietnam, for its troops' brief incursion into Thailand from Cambodia one month ago. It was also to show the Thai public that the government can be firm.

The border closure has hurt badly the already languid Laotian economy, according to reports reaching here. The country's communist government has responded by staging demonstrations in the capital and warning that Thailand would "bear all responsibilities" for whatever happens next.

Earlier this week, Loas sent a Cabinet-level delegation for talks in Bangkok. But in three days of sometimes heated discussions the opposing sides apparently accomplished little in reversing the current mood of confrontation. The Laotians returned to Vietiane yesterday.

The dispute with Laos comes as Cambodia's Heng Samrin government, installed by Vietnamese troops 19 months ago, has been accusing Thailand of repeatedly shelling and overflying Cambodian territory. Adding to the tensions in the area are Chinese and Vietnamese gunners trading artiliery barrages across their common border.

Indochina watchers here remain uncertain to what extent the renewed tension along all three borders is related. Hanoi has alleged that it is part of a larger effort orchestrated by the United States and China to pressure Vietnam and its allies in Cambodia and Laos. The United States and China is backing the Khomer Rouge opposition led by Pol Pol in Cambodia.

The current Laotian government took power in 1975. Supported by Vietnamese forces, the Laotian Communists had overwhelmed an American-supported administration, which relied heavily on irregulars recruited in Thailand and U.S. air support from Thai bases.

Suspicions remain strong between Laos, with a population of 3 million, and Thialand, with its 45 million inhabitants. Nonetheless, the two countries last year signed an agreement to build "genuine peace, friendship and mutual benefit" along the Mekong River.

Thailand granted a $5 million commodity laon, as Laos continued to supply electricity to its neighbor from a dam built before the communist victory. The resulting income reportedly account for almost a third of Laos' foreign exchange earnings.

However, the current standoff appears to threaten this progress seriously.

Vientiane maintains that on June 14 its militia engaged Thai bandits in a riverside village and killed one of them. The next day a Thai gunboat patrolling the Mekong attempted to recover the body as it lay on the Laotian bank.Laotian soldiers opened fire.

Thailand, however, claims its boat was shot in Thai waters without provocation. One Marine aboard died and two were wounded.

Loas rejected Thai demands for an apology and compensation, and two weeks later Thailand halted all traffic except flights across the border. Depending on Thailand for imports, food and transit from the sea. Vientiane viewed this as a hostiale act.

Foreigners in the Laotian capital report the price of certain types of rice has doubled and aid projects that depend on equipment deliveries have been postponed. Gasoline shortages have reduced the numbers of cars on the streets and forced the cancellation of some domestic flights.

The official Laotian news agency last week reported that 10,000 people rallied in Vientiane to denounce "the Peking reactionary ruling circles in collusion with the U.S. imperialists" who were "using a group of Thai rightists to provoke Laos."

The demonstration came shortly before ministers from Laos, Vietnam and the Heng Samrin government in Cambodia began conferring in Vientiane. A communique issued after the talks linked the border closure to Thailand's decision to repatriate Cambodian refugees into border zones controlled by Khmer Rouge guerrillas and to the frequent exchanges of artillery and small arms fire across the Sino-Vietnamese frontier in the past month. China was blamed for instigating that fighting. The communique also proposed that Bangkok and Phnom Penh work together to create a demilitarized zone along the Thai-Cambodian border and arrange for the orderly return of Cambodian refugees in Thailand.

Thailand rejected the proposals since it does not recognize the Phnom Penh government and issued its own charges of a larger conspiracy. A Foreign Ministry statement said that "Laos has demonstrated to the world that it has been dancing to the rhythm dictated by people behind it," a clear reference to the Vietnamese.

In meetings with foreign ambassadors, Thai officials maintained the Vietnamese had planned both the Mekong shooting and the strike into Thailand, launched eight days later. Thailand reported 22 of its soldiers were killed in the incursion.

Some diplomats questioned whether Thailand really believed the two incidents were coordinated and speculated that it was making Laos pay for Vietnam's sins.

"If you don't like a guy but he's bigger than you, then you kick his dog," diplomat commented.

Thailand has already stopped grain shipments to Vietnam. Unwilling to suspend diplomatic relations or make a military gesture of its own, it settled on blockading Loas to make its feelings known to Hanoi, these observers believe. Vietnam will have to shoulder an extra burden of economic aid to Laos, according to this reasoning.

Closing the border might also satisfy Thai public opinion for a firm stand against Vietnam and its allies. Thailand's sensationalist press has given heavy play to the Vietnamese incursion and Thai leaders' pledges not to compromise on Laos.

Thailand's decision suggests it has cast aside the strategy formulated by former prime minister Kriangsak Chamanan of wooing Laos away from Hanoi by strengthening economic cooperation.

But analysts noted that if the blockade succeeds in slowing Laos' economy, Thailand could suffer a quickened flow of "economic refugees." Almost a quarter million Laotians have fled across the Mekong since 1975, many of them more because of unemployment and the country's low standard of living than political differences with the government.