Back in the summer of 1972, the David I. Hitchcock family, like millions of others, took a great cross-country American vacation.

First it was a couple of weeks in a New Hampshire cottage, then a drive in a rented car to Denver for a tour of the U.S. mint and zoo there. Next, it was on to Wyoming for a week at the Trail Creek Ranch resort and, as a holiday finale, a brief visit to Yellowstone National Park.

It was not an unusual vacation, but six months later, Hitchcock took the highly unusual step of claiming his part of the holiday costs - $950 - as a U.S. income tax deduction.

The Internal Revenue Service rejected his claim and was upheld by the U.S. Tax Court in Washington, but yesterday, the 4th U.S. Circuit of Appeals in Richmond ruled that the Hitchcock deduction was legal because he was a foreign service information officer obeying a congressional mandate that he "re-Americanize" himself every 18 months.

The appellate ruling apparently opens the way for thousands of foreign service officers and employes of the International Communications Agency (formerly the U.S. Information Agency) who are stationed abroad to deduct the costs of all future U.S. vacations taken under the mandatory home leave law.

Yesterday's ruling is the second from a U.S. appellate court upholding such a deduction, and a Justice Department tax lawyer said the case probably will not be appealed to the Supreme Court, although that is still possible.