WHO WOULD HAVE imagined that, barely three years after the American collapse in Vietnam, the victorious communist regime would be pleading for political and economic ties with the United States? It's plain enough how this turnabout has taken place: Vietnam, having moved swiftly (and rashly) from war with neighboring Cambodia to dangerous friction with Cambodia's powerful patrons in Peking, finds Soviet support ardent but inadequate and hopes that the United States can plug the gap. In addition, the Vietnamese need the sort of large-scale cooperation in reconstruction and development that United States can best provide.

So it is that Hanoi has dropped its demand that Washington pay "reparations" as a condition of political relations. To a recent congressional delegation searching for (and receiving) the remains of additional American servicemen, it expressed its urgent desire to restore - better, to create - normal ties.

Many Americans see good reason to respond in kind: some to make amends for past American policy; others to cultivate the "Titos," or national communists, of the world; others to make a buck. Some Americans may even feel Vietnam should be unleashed on the murderous regime in Cambodia.

Yet the Carter administration, which came to office promising early reconciliation with Vietnam, is holding back, for considerations variously understandable and substantive. The understandable consideration is that early normalization with a country with which the United States so recently warred would provoke political resentment - without due diplomatic compensation. The substantive consideration is that it would complicate American relations with China.

Let us expand on that point. China's opening to Washington, initiated by Mao Tse-tung in 1972, came under review in the struggle over his succession. But during the summer the Chinese apparently decided to press the opening with a vengeance to counter pressures from Moscow. The basic explanation no doubt lies in the arcane mysteries of Chinese politics. Alert American diplomacy, represented by Zbigniew Brzezinski in his trip to Peking last May, may also have played a role.

Certainly China needed no urging to rally other nations to stand up to the Kremlin. This summer, however, China has consummated its relations with Japan with a peace treaty. Party Chairman Hua Kuo-feng has demonstratively circled the Soviet frontier, visiting the independent-minded states of Yugoslavia, Romania and Iran. And Peking moved to expand its scientific, educational and commercial links with the United States - and its military-supply links with the United States' European allies. Moreover, it has done this without, so far as we know, extracting from the administration diplomatically excessive and politically dangerous concessions on Taiwan.

It is evident that the United States must manage its relations with Peking with care, especially so as not to provoke the Russians unduly, whose cooperation is essential to keeping the peace. We do not happen to think the administration has gone too far with China. What is beyond cavil, though, is that the United States has no interests in Hanoi even faintly of an order with those it has in Peking. That alone is reason to tread water on Vietnam, while the larger question of China is worked out.