VIETNAM'S communist rulers, nearly 13 years after extending their power from north to south, are making their most concerted effort to find a more normal position in the world, but they are proceeding so slowly and hesitantly that the outcome is anything but ensured. The irony of it is the extent to which progress hinges on the United States, the country they defeated in the war.

Evidently Nguyen Van Linh, the new party secretary, has been examining the disaster of his economy, noting the accomplishments of free-enterprise Asian neighbors and perhaps peering at China's bold economic experiment and listening to his impatient creditors in Moscow. Vietnam is starting to dabble in Gorbachev-type economic reforms and to troll for foreign investors. While thousands of prisoners are still in "reeducation camps," blue jeans can be seen in Hanoi. But though its neighbors are trading more with Vietnam, they are restrained by its continuing occupation of Cambodia. Honda was ready to open a motorcycle project; American pressures, among others, killed it.

In a welcome political initiative, Hanoi's puppet in Phnom Penh, Hun Sen, has just met twice with Prince Sihanouk, veteran leader of the weakest resistance faction. But the murderous Pol Pot, the principal resistance figure, continues pressing in what is not only a civil war in Cambodia but also an intense proxy struggle between the People's Republic of China, supporting Pol Pot, and the Soviet Union, supporting Vietnam. The United States demands that Vietnam go home. Relief awaits Sino-Soviet developments, barring a (surprise) decision by Vietnam to leave Cambodia on its own.

The immediate and abiding American concern is an accounting of the 1,700 Americans still listed as missing in action in Vietnam. President Reagan sent Gen. John Vessey to Hanoi last August, and he returned with an understanding that Vietnam would help resolve MIA cases and the United States would encourage private organizations to send humanitarian assistance -- for Vietnamese who lost limbs in the war. Hanoi undercuts this modest arrangement by delaying on MIAs and repeatedly injecting a demand for direct American aid, something that cannot even be considered while the MIA and Cambodia issues fester.

Vietnam, however, has just agreed to restart the emigration of willing Amerasian children and their families -- a much-abused and deserving group. This is the road on which it must go much farther for an accommodation with the outside world.