The leaders of Vietnam and Laos signed three treaties today, giving formal status to the close relationship that has developed between the two Communist neighbors since the end of the Indochina war.

The accords, one of friendship and cooperation, a second providing for three years of Vietnamese economic assistance to Laos and the third defining the Vietnamese-Laotin border, were signed in Vietiane by the Communist Party leaders and prime ministers of the two countries. They also signed a joint communique.

The details of all four documents are expected to be released Tuesday by the official media of the two nations. After a signing ceremony today, the Vietnamese delegation left Laos for Hanoi, Vientiane Radion announced in a broadcast montiored here.

The signing of the treaties was expected to increase the anxieties of the non-Communist countries of South-east Asia, who fear that an armed alliance may be formed by the three Indochinese states for external aggression. The likehood of Cambodia joining such a group is remote, given the xenophobic character of the Phnom Penh government and its closeness to China, but the new Vietnamese-Laotian accords attest to their bilaterial unity and, in turn their alignment with the Soviet Union.

The Vietnamese delegation, led by Communist Party General Secretary Le Duan and Prime Minister Pham Van Dong, was the highest-ranking team ever to travel abroad. The Laotian side was led by Prime Minister and party general secretary Kaysone Phomyihan.

In speeches during the four-day visit, leaders of both sides stressed closeness of their alliance. Last night, for example, Le Duan told guests at a farewell banquet: "We were very glad to note that the two sides saw eye-to-eye on all questions. The joint statement to be released soon will be a brilliant expression of the especially pure and extremely solid relations between our two parties and peoples."

Most of the remarks were couched in socialist diplomatic jargon, and both sides stressed the "militant solidarity" of their relationship.

While "firmly pursuing an independent and sovereign line and at the same time respecting the independent and sovereign line of their friends," Le Duan said at another banquet Friday night, "the two countries, which have long been bound together forever in the cause of socialist construction and national defense."

Whether any of the documents signed at Vietiane's government guest house deals specifically with defense is not yet known. There are now between 40,000 and 50,000 Vietnamese troops in Laos.

Their presence is a source of continuing concern to the anti-Communist government of Thailand, just across the Mekong River from Laos, and the other non-Communist nations of Southeast Asia.

Several thousand Vietnamese soldiers are known to be building roads. Others are understood to be assisting Pathet Lao forces combating Meo tribal resistance.

Vietnam and Laos repeatedly charge that Thailand is arming and assisting the Meos. The Communists have also implied that the United States is behind the armed resistance.

Thailand, in turn, says that since the end of the war in Indochina, the Vietnamese have been channeling arms and other supplies to Thai Communist insurgents in the country's northeast. Similar, but softer, charges that weapons from Vietnam are making their way to Communist insurgents in northern Malaysia have also been made by officials in Kuala Lumpur.

At a recent meeting of foreign ministers from the five-member Association of Southeast Asian Nations, Singapore's prime minister, Lee Kuan Yes, warned his colleagues that non-Communist Southeast Asia had "to live with a continous land mass which is Communist, all the way from Siberia and China down to Vietnam, Laos and Kampuchea," or Cambodia.

During a visit to Hanoi early this year, Lotian leader Kaysone acknowledged that Laos relied on Vietnamese military assistance during the Laotian var, first against France and then against the U.S. - backed forces. The Vietnamese, he said, "had sent their sons directly to take part in and fight to assit the Laotian revolution."

Kaysone's frankness contrasted with the Pathet Lao's insistence during the protracted war that Vietnam played no active combat role in Laos.

According to Vientiane Radio, the Laotain government spared nothing in honoring their Vietnamese guests.A hundred, thousand Laotians turned out to welcome the delegation when it arrived last week and "tens of thousands of Vientiane residents" lined the road to the airport as they departed this morning, the radio reported. The sleepy riverside city has a population of about 70,000.