Union officials at the Census Bureau have been taking a leaf from the investigative reporter's notebook.

Using the Freedom of Information Act, they obtained dozens of travel vouchers for top Census officials, who they believed were junketing around the world at a time when 300 bureau employes lost their jobs.

Next, Local 2782 of the American Federation of Government Employes published the results in a newsletter seething with indignation. And to make sure its findings did not go unnoticed, the union sent copies to congressional offices and the news media.

AFGE's first installment focused on former Census Bureau director Bruce Chapman, now director of the White House Office of Planning and Evaluation. The vouchers show that Chapman took 17 out-of-town trips from August 1981 to August 1982, including visits to Europe, the Middle East and South America.

In the best tabloid tradition, the union picked the sexiest item on Chapman's expense account -- a couple of five-dollar shoeshines in New York -- for the story's headline. And its rewrite man stressed that Chapman had spent more than $10,000 on travel while 750 employes, or 25 percent of the bureau's staff, were being laid off, downgraded or transferred in a reduction in force.

"As taxpayers, we are rightly offended to find that in these times of monumental deficits, and during the biggest RIF in the federal government, our taxes were used to pay for the director's shoeshines and laundry," union president Edward Hanlon said in his press release.

Chapman said the criticism is unfair and unfounded. He said his foreign trips helped win millions of dollars in new contracts for the bureau's international program, which studies overseas trends and provides training for foreign statisticians.

"Every one of those trips is justified," Chapman said. "It's frequently helpful to have a high-level person to deal with the foreign statistical agency. If you're going to develop a program which in the end is going to add jobs to the Census Bureau, I don't see how you can be criticized for that."

The RIF was handled "as humanely as possible" and some employes have since been rehired, Chapman said.

He said he saw no problem in occasionally billing the agency for such expenses as shoeshines and magazines because he was told that "you put in everything you did that is pertinent to the trip."

On the Middle East trip, Chapman said, he met with officials in Saudi Arabia and flew to Africa for the Joint Conference of African Planners, Statisticians and Demographers. He said the trip was covered by the Saudi government's contract with the bureau.

Chapman said he visited Buenos Aires to present a paper at the International Statistical Institute. He said he took an overnight ferry from Uruguay because it was no more expensive than staying at a hotel.

Some of Chapman's trips were paid for by outside groups. His visit to Brussels, Paris, Bonn, Luxembourg and Dublin, for example, was paid for by the Common Market's governing body. Chapman said he received the invitation before joining the Census Bureau and "decided to take it as a vacation."

Chapman said his two summer trips to Seattle, his hometown, were not "a thinly disguised vacation," as the union newsletter had described them, and that he charged expenses only while conducting official business.

AFGE officials were particularly upset that Chapman took 20 aides to a statistical meeting in Canada in 1982. Chapman said the contingent was smaller than usual because of budget considerations.

In one expenditure missed by the union sleuths, Chapman billed the government $135 to attend the Conservative Political Action Conference at the Mayflower Hotel here. He said he repaid the money after the General Accounting Office questioned it.

The AFGE is planning other journalistic forays. It has already reported that the new Census director, John Keane, has visited Europe and China since taking office earlier this year.

Census spokesman Jim Gorman said that Keane signed a cooperative agreement in China and met with statistical officials in Britain and Germany. He added that travel by Census officials "is not excessive."