John Vorster reentered public life yesterday.

For the former prime minister and state president, whose involvement in covering up a government scandal gave South Africa its equivalent of Watergate, the step back into the limelight was perhaps more revealing of the man's personality than of his potential for political revival.

Thursday he addressed an overflowing hall of more than 2,000 students at the conservative University of Pretoria.

It was clearly not the start of a political comeback. Vorster stands no chance of regaining high office. He appears rather to be intent on settling some old scores with his successor, Prime Minister Pieter W. Botha.

Vorster's bitterness against Botha is widely known in political circles here. It was Botha who forced Vorster's resignation over the so-called Muldergate scandal, after Connie Mulder, the former information minister who misused a huge slush fund Vorster allowed him for running propaganda operations.

According to his friends, Vorster claims that Botha was as knowledgeable about the misuse of the funds as was Vorster.

In his speech yesterday, Vorster declared his opposition to Botha's token attempts to reform apartheid, the South African policy of racial separation, thus lending his support by implication to the extreme Conservative Party that broke away from Botha last year.

If implemented, he said, the Botha plan "will sound the death knell of the white man in this country."

Afterward Vorster was met by a beaming Andries P. Treurnicht, the Conservative Party leader, who told reporters with smiling understatement: "I think there was some comfort in that for me."

In a high-passion dispute that has split the once closely knit Afrikaner community, Treurnicht is locked in an electoral duel with one of Botha's senior ministers, Stephanus P. Botha. Local commentators agree that if the prime minister's man loses the May 10 contest it could be fatal for the reform policy and crippling for Pieter Botha. If Treurnicht loses it will be the end of him and his party.

With stakes like that, Vorster appears to hope his intervention as a one-time folk hero among the white Afrikaner--he won the biggest election victory in the country's history two years before his fall--will swing the balance against Botha.

Vorster's aid to the conservative cause hardly seemed convincing to anyone with a memory of Vorster's 12-year premiership. He was never on the right wing of his National Party.

In fact, he initiated the pragmatic revisionism that Botha is now taking a little further, and the first breakaway by an extremist splinter group, the Herstigte Nasionale party in 1969, occurred under his tenure.

It was never any secret that he did not share Treurnicht's hard-line views, and he gave Treurnicht a nominal Cabinet post to hobble him.

Now, after nearly four years of political obscurity spent living in an apartment overlooking a motorway in the out-of-the-way city of Port Elizabeth, Vorster has taken an unusual opportunity to seek revenge.

Yesterday's speech was charged with emotion, leaving his audience alternately hushed and cheering.

The essence of Vorster's message was that Botha, by proposing to extend even token parliamentary representation to the Asian and mixed-race (colored) minorities while still excluding the black African majority, was crossing a Rubicon on the power-sharing issue.

"If you lose political power, then you lose everything of yours that is valuable and meaningful," Vorster said.