BANGKOK — KEYSTONE countries of war-torn Indochina are taking a first step in what will likely be a slow return to the world economic mainstream.On Oct. 23, Cambodians will sign a peace accord in Paris, ending more than 12 years of civil conflict. That will open the country - which over the last three decades has been shattered by American bombing, mass executions, invasion, and war - to international trade and financing. The United States says talks to normalize relations with Vietnam will begin once Cambodia's transition to peace is under way. Laos already is a jump ahead of its neighbors, receiving major international financing and moving toward economic modernization. However, both Vietnam and Cambodia are unlikely to win substantial funding soon, financiers say. With a 16-year-old American economic embargo still stubbornly intact, a French bid to win financial support for Vietnam fell flat at the recent annual meeting of the World Bank and the International Monetary Fund (IMF) here in Bangkok. Vietnam has been isolated for almost 40 years by wars against France and the US and a 10-year proxy war against China in Cambodia. The US, which at one time pegged new financing to a Vietnamese withdrawal from Cambodia, now says it wants free Cambodian elections under a peace plan sponsored by the United Nations and cooperation on Americans missing since the Indochina war. Many Western financial leaders doubt the Bush administration, grappling with a troubled US economy and domestic touchiness about Vietnam, will lift the blockade before presidential elections next year. France leads a growing number of countries that want to help Vietnam clear its overdue debt and qualify for new funding for an economy battered by years of war, communist mismanagement, and the recent cutoff of Soviet aid. For Vietnam's beleaguered economy, the immediate roadblock to more aid is arrears of $140 million owed to the IMF since debt repayments were interrupted in 1985. During the last three years, the country has kept current on payments. Officials of the State Bank of Vietnam project the country will need $600 million in foreign loans for a three-year program to cover back debt payments, restructure the economy, rebuild a decrepit infrastructure, and lure new foreign investment. Cao Si Kiem, governor of Vietnam's central bank, told the meeting of financiers in Bangkok that "despite limited export earnings and ever-increasing import needs, we have made efforts to earmark part of our small foreign exchange for servicing our debt to external creditors." Market reforms, launched in 1986, also continue despite the lack of progress on removing the embargo and slower economic growth. The government, which projects growth next year will fall to 2.5 percent from 3 percent in 1990, announced plans to end subsidies to state enterprises and expand the private sector. "Time is of the essence if they're going to carry on the reform program. The next steps are difficult without a funding source," says Gautam Kaji, an official of the World Bank, which provides technical assistance and analysis to Vietnam. "Once Vietnam reenters the fold, I think the interest [by investors] is going to be very very significant," Mr. Kaji said, explaining that the World Bank also is ready to move "briskly" once the embargo disappears. Yet, to reinforce its opposition, the US last month warned some non-American international banks to stop handling dollar transfers to Vietnam. That forced Vietnam to suspend use of the dollar for international transactions. Economic observers say Washington is increasingly isolated as Vietnam enjoys growing trade and investment ties with its noncommunist neighbors and others. Countries such as Australia have announced resumption of direct aid to Vietnam. Even Japan, which has staunchly backed the American embargo, is showing signs of flexibility although it may not be ready to risk an outright confrontation with the US (See related story, left). For Cambodia, finance officials will be starting from scratch. In arrears to the IMF for about $50 million, Cambodia stopped paying during the brutal four-year rule of the Khmer Rouge in the 1970s. The radical Marxists imposed a chaotic agrarian restructuring on Cambodia and destroyed the monetary system. In recent years, the Phnom Penh regime headed by Hun Sen liberalized the economy somewhat. Cambodia is expected to require billions of dollars in aid and financing for reconstruction. But first, officials say, the interim Supreme National Council will have to restore the financial infrastructure and analysts must assess investment needs. That could take a year, they say, with actual funding not starting until after elections proposed for early 1993. The US says it will lift trade sanctions against Cambodia once the peace accord is signed.