For South African Prime Minister John Vorster, this week's meeting with Vice President Walter Mondale marks the most serious challenge in his political career.

Vorster's position during the Vienna talks with Mondale will mark a turning point, for the strong-willed South African leader must finally make it clear whether Western support for his controversial government is worth the stiff price tag: changing South Africa's basic political structures to allow "full participation" by the majority black population.

Vorster has made it clear in public that these talks will be different from last year's two meetings with former Secretary of State Henry Kissinger. This time there will be no option of buying time for South Africa in exchange for settlements in Rhodesia and Namibia (Southwest Africa).

The change in approach by the U.S. administration appears to represent the most serious threat from the West to date, and Vorster's concern has been increasingly conspicuous.

With each new report of what U.S. officials will say to him, the prime minister or one of his Cabinet officials has stated that the South African government intends to stand behind its homelands policy - the division of the country over the next few years into nine small African states and one large white one.

On the eve of his departure, Vorster told a nationwide television audience: "In some circles it is suggested that I am going to Vienna to take orders. I'm not going there to take orders . . . I will make use of the opportunity to put South Africa's case to Mr. Mondale - and South Africa certainly has a case to put.

"South Africa has made its stance clear, on all occasions, that it is willing to cooperate to find peaceful solutions. But no self-respecting country can allow itself to be pressured in certain directions."

In an interview with an Austrian publication this week, he used even stronger language: "We will never surrender. We will fight for our land. South Africa is an independent country and certainly nobody from the outside can dictate how it should run its country or its domestic affairs."

This defiant stance has led South African political observers to speculate that even relatively attempts at persuasion by Mondale will lead Vorster to withdraw into an isolationist shell.

The Johannesbury Star predicted a confrontation that "could lead to a South Africa closed to outside influences with an end result opposite to the peaceful solution to southern African problems now so urgently being sought by the United States and the West."

Western diplomats here have tried to push a more hopeful line, saying that Vorster has displayed new interest in helping find internationally acceptable terms for establishing a black government in South Africa-administered Namibia.

During recent negotiations with representatives of five Western nations - the United States, Britain, France, West Germany and Canada - he opened the door to free elections, under outside supervision, before Namibian independence.

There are also growing indications that he is willing to pressure Rhodesian Prime Minister Ian Smith into accepting the terms of the latest Anglo-American settlement plan to establish a black-majority government there next year.

The diplomats also acknowledge, however, that obtaining concessions on greater black participation in South Africa itself will be more difficult. Part of the problem, according to one Western official, is South Africa's overestimation of its value, strategically, economically and politically.

"For the talks to go well, the whole outlook of the South Africans will have to be changed," one diplomat explained. "Firstly, their perception of their strategic interests is wrong and outdated. That 1950s attitude of fighting communism by aligning with the pro-west groups isn't accepted any more."

The argument about protecting the sea route the Cape of Good Hope also does not work, the diplomat said, adding, "It is not necessary for most vessels to stop at the Cape. Those that need to stop can do so in Maputo (Mozambique) or Luanda (Angola), though it is more expensive."

A recent survey by an American economic analyst has also inducated that South Africa is not essential to the United States for its mineral wealth.

According to government officials here, Vorster will attempt to counter the discussions of change by pointing out that his Cabinet is reviewing the country's system of government - to allow for a new role for Asians and "Coloreds," as people of mixed race are called here but not for blacks - and that a bill in Parliament would give blacks greater powers in their townships.

He is also likely to bring in the fact that, since the racial unrest in June of last year, his government has promised more money for African education, moved toward allowing home ownership for blacks, and held a series of negotiations with black leaders.

In a South African context, these are viewed as significant moves, and the prime minister is likely to argue that he need more time to introduce other changes to improve conditions for blacks without alienating his white constituency.

He will be arguing on a different level from that Mondale intends however, because his "changes" amount to moves on social discrimination, leaving the political structure intact.