IT IS HARD to exaggerate the importance of the passage that southern Africa has entered by virtue of John Vorster's resignation as prime minister of South Africa and South Africa's simultaneous rejection of the U.N. plan, which it previously accepted for bringing independence to Namibia. Mr. Vorster's departure removes a leader who, is stubborn and perverse by Western standards, had the stature at home to make serious change at least thinkable, no possible successor can hope to claim the same power. Meanwhile, Pretoria's rejection of the U.N. plan on Namibia creaters a new prospect of crisis: In place of reasonably orderly, internationally approved change in Namibia, internal violence and international tension hang ever more darkly over the whole southern African sky.

What went wrong with a Namibian plan that seemed so valuable and promising to South Africa just a few months ago? South Africans claim that the United Nations, to favor the externally based guerrilla group know as SWAPO, made unacceptable changes in the April plan: increasing the size of the U.N. election-supervising force to 7,500, installing a 360-man U.N. supervisory force over the local police, pushing back the date of elections by a few months. But it is absurd and unforgivable for South Africa to be quibbling over changes, if they are that, of such trivial dimensions, when quibbling means putting at risk the immense gains in security and respectability ensured by sticking with the U.N. plan. It is hard not to conclude that Pretoria's action arose not from these details but from pressure among Mr. Vorster's would-be successors to show their toughness at a moment of transition.

This sequence is foreseeable if Pretoria runs its own elections: First, SWAPO will resume guerrilla war and invite in the Cubans, with all that means to the American effort to provide a peaceful non-communist option for the settlement of African disputes. Then, the United Nations will vote economic sanctions against South Africa, facing the United States with an issue painful and divisive in both its political and diplomatic aspects. Further perhaps deadly blows will be delivered to attempts to bring about a decent solution in Rhodesia and to substitute reform for revolution in South Africa itself.

With stakes of the magnitude, there is good reason to proceed with the utmost care. Hoping against hope, the United States is taking the position that it is still possible to draw back from the precipice. It is trying to hold the decision open and give Pretoria room to reconsider. The fight over the succession makes this an extraordinarily difficult time for South Africa to do that. Yet if South Africa cannot, there is scant reason to believe it can do anything effective to spare itself a storm dwarfing every other ordeal it has endured in the past.