Vietnam has just either locked tiny Laos into a quarter-century of servitude or formally stated its concern over the extent to which it must shore up its weak neighbor.

Although most Western observers of the Indochina scene here in Bangkok subscribe to the servitude thesis, some believe that a 25-year friendship treaty Laos and Vietnam signed this week is an expression of Vietnamese worries rather than a declaration of colonialization.

"Hanoi certainly has enough to do in its own territory without wanting to divert men and material aid to Laos," commented a Western diplomat whose government maintains an embassy in Vietnam's capital.

"The treaty may be nothing more than the Vietnamese going on record, for their own enlightment as well as the Laotians', on how deeply they've committed themselves."

He noted that right now, "in a time of peace," Vietnam has about 40,000 troops in Laos, when, he said, "It would almost certainly prefer to use these men to either put down any uprisings in southern Vietnam or put them to work opening up new farmland and on other developmental projects at home."

Another Western diplomat saw the other side of the coin. "There's no way the Vietnamese can gainfully employ their entire army at home," he said, "so they're obviously only too happy to put them to work making sure the Laotians do exactly as they're told."

The bulk of the Vietnamese troops are believed to be occupied building roads within Laos and linking the landlocked nation to Vietnam's seasports as well as combating a nagging insurgency, by anti-communist Meo tribesmen.

But the treaty indicated that Vietnam, with its army of 1 million, was prepared to fight for Laos' army of 12,000 to 14,000 in the event of another war, as it did in the past.

"Both sides pledge to wholeheartedly support and assist each other and to closely cooperate in increasing the capability of defending the indenpedence, soverignty and territorial integrity" of each other, the treaty stated.

At the same time, Vietnam committed itself to provide Laos with considerable civilian administrative expertise and an undetermined amount of economic aid. The aid was formalized in another document signed by leaders of the two countries last week.

Described in a joint communique and free loans for three years," the aid treaty will take effect next years the communique released by the Vietnam News Agency.

No amount was made public and the aid treaty itself has not been released. Neither has a border delimitation treaty signed at the same time on Monday, at the end of a four-day visit to Laos by a delegation of Vietnam's top leadership.

Western Vietnam-watchers in Bangkok said they are not aware of any outstanding border problems between the two Communist neighbors, but much of the frontier is believed to be ill-defined.

The joint communique said that the two governments are determined "to build the Vietnam-Laos borders into frontiers of lasting brotherly friendsjip." A more cynical view, expressed by several Western diplomats here, is that the Vietnamese want to assure themselves of an open frontier for troop movements.

Most diplomats observers agreed, however, that Vietnam could not be too happy about diverting any amount of economic assistance to Laos. International donors have promised Hanoi more than $6 billion over the past two years, but Western economists believe that all of it is required for reconstruction and basic development.

Indicating that financial assistance is still vitally needed by both countries, the joint communique reminded the United States of its Nixon-era promise to "heal the wounds of war."

Both countries suffered grave physical looses from U.S. bombing during the war in Indochina and deficiecies resulting from being unable to build their economies during more than 30 years of war, first against France and then against the United States.

Thus, Article 3 of the 25-year friendship treaty asures a broad range of exchanges "to create advantages for each other and to help each other overcome difficulties and effectively develop each country's material potentialities." Areas to be covered include agriculture, forestry, industry, communications and transport, and there will be a number of other technical, economic, cultural and scientific exchanges.

With Vietnam's population at 50 million and Loas' just above 3 million, clearly the Laotians stand to gain far more than they can give. In economic terms, then, Vietnam's intentions would seem to be more those of big brother than colonialist exploiter.

But in terms of the two states' relations, with their non-Communist neighbors in Southeast Asia and with the United States, the treaties show that there is less for the West to be optimistic about.

"The two sides severly condemn the United States for maintaining its troops and military bases in South east Asia and attempting to use the Association of Southeast Asian Nations to oppose the trends for real independence, peace and neutrality in this region," the joint communique stated.

The document specifically attacked Thailand and its anti-Communist government for carrying out policies allegedly hostile to Vietnam and Laos since the military siezed power in a coup last October.