Laos and Vietnam, whose Communists formed the backbone of a revolution that defeated superior U.S. firepower 14 years ago, are showing signs of drifting apart, according to Western diplomats and Laotian officials.

Differences over economic cooperation have highlighted strains in the special relationship between the two, forged during years of struggle against French colonialism and U.S. anti-communism, they said.While public statements by the two Soviet allies still eulogize the "everlasting bonds" between their peoples, their mutual dependence seems to be waning as both look increasingly to non-Communist states for aid and trade.

"They promised us everything, but words come cheap," one Laotian official said of the Vietnamese. "They haven't delivered."

Laos is increasingly turning to its natural trading route to the sea through its ethnically related southern neighbor Thailand.

It has taken years to build a suitable road for commerce through the mountains that run like a spine down the Indochinese peninsula and give access to the Vietnamese port of Da Nang.

On the other hand, plans are quickly crystallizing for a new bridge over the Mekong River to link with the superior road network to the Gulf of Thailand.

One of the Communist world's poorest and most reclusive states, Laos has relied heavily on Soviet aid and Vietnamese support since the two Indochinese countries became Communist in 1975.

But unlike in Vietnam, neither China nor the United States closed down its embassy in Laos despite souring relations and sporadic exchanges of accusations and recriminations.

Both Indochinese countries have implemented liberal reforms and sought closer relations with the non-Communist world. But diplomats said Laos has not been hindered by some of the constraints Vietnam works under, like Hanoi's 10-year military presence in Cambodia.

"These are problems they (Vietnam) have had but not aired before," one Soviet bloc official based here said. "There's a strong feeling that the younger brother (Laos) shouldn't lead in these matters."