Lee accepted the resignation of Kim Hyo-jae, his senior political affairs secretary, on the weekend after it became known that prosecutors wanted to interview him.
Kim is to be interviewed Wednesday over alleged involvement in the scandal during the ruling party's 2008 leadership election which claimed the head of another politician last week, Yonhap news agency said.
Kim allegedly was a key player in the scandal in which envelopes of money were allegedly distributed to Grand National Party politicians before their colleague Park Hee-tae was elected as party leader in 2008, Yonhap said.
In 2010, Park left his job as leader of the party -- now called the Saenuri Party -- to become speaker of the National Assembly.
Park won 236 out of 249 votes in February 2010 to land the two-year job, a report by the Chosun Ilbo newspaper said at the time. Park was born in Namhae, South Gyeongsang province and is a law graduate of Seoul National University.
In 1988 Park became spokesman of the Grand National Party and was re-elected six consecutive times. He became minister of Justice in 1993, before taking over as speaker.
But Park, 74, resigned as speaker early last week, claiming responsibility for the vote-buying scandal, Yonhap said.
Park's resignation was followed by that of Kim, who worked in Park's 2008 election organization. Kim joined the president's office last year and has denied any involvement in the alleged bribery, the Yonhap report said.
"President Lee accepted Kim's resignation offer but he made no special comment on this," a presidential official said on the weekend.
Kim, a former editor with Chosun Ilbo, is one in a long line of journalists who have moved into political advisory roles only to become victims of scandals, an opinion piece in the independent newspaper Hankyoreh said.
"Reporters who cozied up to powerful politicians are going down with them," said senior Hankyoreh staff writer Kim Do-hyung. "The very press figures who should be cleanest of all are leading the way in illegalities and corruption."
Only Lee Dong-kwan, a former political reporter for the Dong-A Ilbo newspaper and now a senior secretary for presidential public relations, remains unscathed among Lee's associates from three conservative newspapers, he said.