THE UNITED STATES could have sent a lesser figure to Park Chung Hee's funeral, but it is sending Secretary of State Cyrus Vance. His mission announces an intent to go beyond paying respects to South Korea's slain president and to convey to Seoul, which is unquestionably awaiting just such a signal,, what American hopes for Korea's future are. The evident message he will convey is that the United States is devoted not only to South Korea's security but also to its careful but unmistakable progress toward a more open form of government.

For years Korea has been in the middle of the American debate over how to balance the pursuit of stability and the pursuit of democracy around the world. It has been argued, for instance, that modernizing states -- especially those, like Korea, under unremitting external challenge -- lack the tradition or social structure or internal leeway to support democracy, and so the United States would be better off settling for what it can get. Analyses of this sort are often accompanied by homilies on the virtue of a great power's recognizing the limitations on its capacity to manipulate foreign societies.

The Carter administration has provoked some of these homilies. To Third World ears turned only irregularly to Western discourse, American words designed to give more elbow room to the local political opposition may sound like coded aid and comfort to those who would challenge the regime. Yet Korea must be treated in its own terms. The basic fact is that before he changed the rules in 1972 to ensure his own dictatorial rule forever, President Park ran a passably decent show. True, he came to power in 1961 by coup, but after that he allowed three elections and the opposition was, if not successful at the polls, at least vigorous, and people had some rights.

In brief, there is good reason for the United States to do nothing inadvertently "destabilizing" to South Korea, a country with a neighbor, North Korea, so totally hostile as to make South Koreans of whatever persuasion anti-communist to the core. But the secretary should indicate, too, at this rare moment when Seoul's political future is once again opening, that what the United States wants for South Korea is a system not unlike oe that South Korea has tested and found usuable in its own recent past. The risk to South Korea lies not in adopting, or retrieving, a political system that reflects the extensive modernization of its economic and social structure -- i.e., the growth of a middle class, the spread of education, and so on. The risk lies in sticking with a form of one-man rule rendered obsolete by the society's development and opened up for change by the death of the man who created it in his own name.