CEDAR RAPIDS, IOWA, JAN. 18 -- With the triumphant cry "Fini!" (finished), Vice President Bush today shredded a Rep. Jack Kemp campaign flyer criticizing his record on abortion.

Bush took the one-page handout from a student at Regis High School, a Roman Catholic parochial school here, after she had asked him two questions based on the flyer's assertion that "George Bush has changed his position on abortion three times and promoted abortion while serving as U.N. ambassador." Denying the accuracy of the statements, Bush said, "Let me look at that," glanced over the document and then, with a grin, read the audience of several hundred students, teachers and parents the disclaimer line printed on the bottom, "Paid for by Jack Kemp for President."

When the cheering stopped, Bush said, "I guess we have to write that off as politics. I'm glad you brought it up. I didn't know these things were being passed around." Then he ripped the flyer to pieces, saying "Fini!"

In Washington, Kemp's spokesman, John Buckley, said the flyer accurately represents Bush's differing positions on abortion since he served at the United Nations and wrote a foreword to a book that supported abortion for family planning purposes in the Third World. "His past is catching up with him," Buckley said.

The putdown of Kemp was an enjoyable diversion for Bush from his main task of trying to reverse Sen. Robert J. Dole's (R-Kan.) apparently widening lead in the Feb. 8 Iowa caucuses. Bush described himself to the high school crowd as "the underdog in this state," and his Iowa chairman, George Wittgraf, said he thought the Des Moines Register Iowa Poll published Sunday, which gave Dole a 41-to-26 percent lead over Bush, was "accurate as of last week."

Wittgraf said Bush would spend 10 of the 21 remaining days before the caucuses in Iowa. Rich Bond, Bush's deputy national campaign manager, told reporters aboard Air Force 2 that while an Iowa victory is vital for Dole but not Bush, "We're playing to win" here.

But both said that Dole has special advantages here, not only as a Kansas neighbor but also because Iowa Republicans have heard more criticism of the Reagan administration from the Register and from Republican elected officials than almost any other Republicans in the country.

Campaigning here, Bush criticized Dole's recent endorsement of an oil import fee. He told a Rotary Club luncheon, "Sen. Dole is for it. So are some of the Democrats. I'm strongly against it . . . . It would hurt Iowa farmers and businessmen just when we are getting competitive again in world trade. A $10-a-barrel oil import fee would cost Iowa $668 million a year. This is an anti-business, anti-farmer, anti-consumer tax. It would cost every person in Iowa $230 per year or $625 per household. This tax would devastate Iowa's economy and halt the recovery in its tracks."