VICE PRESIDENT MONDALE went to Vienna to meet South African Prime Minister John Vorster saying that what the United States favored was a change from apartheid to "full political participation" by all races. Afterward, however, he said that full participation meant "one man, one vote." With all due respect to Pretoria's record of twisting words to conceal crudities of policy, therein lies a distinction on which hopes for an easing of the South Aftican crisis may well rest.

Full participation" means, broadly, that all blacks would participate in the political system within a unitary South African state (not just within black homelands) and that whites would not be faced with an immediate demand to deliver themselves to domination by non-whites. Moral principle argues against such a compromise; sound policy argues for it. "One man, one vote," however, is a phrase open to the interpretation that Washington seeks to apply to South Africa's differing circumstances its own fully democratic pattern. The most impassioned white advocates of change in South Africa are not really ready to take such a drastic step all at once.

What Mr. Mondale did by moving from one phrase to the next was to hand the Vorster government a ready-made club with which to beat back the opposition parties' claim that , if South Africa were to move in the direction of the halfway house called "power sharing," then the United States would respond appropriately. Even within the ruling Nationalist Party, there are some slight hints of willingness to consider a federal or confederal scheme in which the special brutalities of apartheid would be reduced. In these hints lie what thin prospects there are to avoid calamity.

The hypocrisy of the South African position is evident: The government abhors the thought of black domination even while it practices an appalling brand of white domination. Still, the United States cannot afford to deny some prospect of recognition and encouragement to those whites who are perceptive and brave enough to contemplate change. It is all very well to insist that over the long haul there can be no moral compromising with a system that is anathema to American values. But, at least transitionally, the Carter administration should be ready to recognize good-faith movement toward a political structure that falls short of the American ideal. Indeed, at this point it would probably be tactically unwise to tell South Africans in advance how far and anything else today is that particular kind of political understanding on the part of this country that would encourage some spirit of accommodation on the part of South Africa. For the beginnings of some sort of movement down the tortuous road of accommodation is South Africa's only conceivable alternative to catastrophe.