Vice President Walter Mondale sought today to deflate reports that he would confront South African Prime Minister John Vorster with a tough new policy opposing apartheid when the two men meet later this week in Vienna.

Speaking to reporters in Lisbon at the beginning of a 10-day swing across Europe that will center on talks on southern Africa, Mondale said he was approaching Vorster "in a constructive frame of mind. We hope for success."

The vice president, who arrived in Lisbon last night, met for 90 minutes this afternoon with Andrew Young, the American ambassador to the United Nations. Young's sharp criticisms and plan to visit South Africa next week have angered Vorster's white minority government.

Mondale, who has been given overall responsibility for African policy in the Carter administration, appeared to be trying to soften the impact of published reports that a series of specific and tough demands would be presented to Vorster.

"We have a policy, but we don't have a program. We are not going with a checklist," an American official traveling with Mondale told reporters after the meeting with Young.

In another portent of a softer tone for the Vorster meeting, Mondale told reporters on his Air Force jet yesterday that he preferred to speak of American commitment to "full participation" in government rather than "black majority rule."

President Carter has endorsed eventually majority rule by South Africa's 16 million politically disenfranchised blacks, but the phrase[WORD ILLEGIBLE] flag to Vorster's National Party, the dominant political group among the country's 4.2 million whites.

Aides to Mondale and Young said after their meeting that they had discussed the American approach to resolving racial and national problems in Rhodesia, in the disputed territory of Namibia that is known as South-west Africa and within South Africa itself.

Young reported to Mondale on last week's meeting of American ambassadors to Africa in Abidjan, the Ivory Coast. He leaves Monday for Maputo, Mozambique, where he will represent the United States at a United Nations-sponsored conference on Namibia and Rhodesia.

The ambassador is scheduled to travel to Johannesburg on May 21, Mondale said today that the disputed trip, in which Young plans to meet with and address private citizens, "is definitely on." As late as yesterday afternoon, a spokesman for Young expressed doubt that it would take place.

Mondale was met at the Lisbon airport last night by Portuguese Premier Mario Soares and nearly all the members of his minority Socialist Party Cabinet. Mondale suggested that his visit to Portugal, which ends Tuesday when he goes to Spain, would center on American economic aid and this country's role in NATO.

He meets Vorster in Vienna on Thursday for two days of talks before traveling to Yugoslavia and Britain.

Today, Mondale's aides emphasized the "the new measure of credibility" that American efforts in Africa had gained under the Carter administration, which they said "is taking Africa seriously."

In his airborne briefing, however, the vice president gave one indication that he is not totally discarding the approach taken by former Secretary of State Henry A. Kissinger toward the multiple crises in southern Africa.

Without mentioning Kissinger's efforts, Mondale said that he would ask for South Africa's "support" in getting a new constitution and free elections in Rhodesia by 1978.

Kissinger reportedly felt he could not pressure Vorster on change inside South Africa while seeking his help in ending the guerrilla wars on South Africa's borders. Mondale has not yet disclosed whether the Carter administration takes a radically different view.

Mondale said he would "clearly describe our policies to the leaders of the South African government. We will discuss the principles we consider to be important to an improved relationship between our government and theirs." He said he would emphasize American "experience in the elimination of discrimination and the establisment of full citizen participation."

Prime Minister Vorster warned in a newspaper interview that other nations would not be able to change the way South Africa handles its domestic affairs, the Associated Press reported.

"South Africa is a independent country and certainly nobody from the outside can dictate how it should run its country or its domestic affairs," Vorster told Vienna's Kronenzeitung.