Virginia Lewis Piper, 65, a socialite who was found shackled to a tree in northern Minnesota after being kidnaped from her home 16 years ago. Two masked gunmen kidnaped Mrs. Piper, the wife of millionaire Harry C. Piper Jr., retired chairman of the investment firm of Piper Jaffray Inc., from her home July 27, 1972. Nearly two days after Piper paid the kidnapers $1 million, then the largest ransom ever paid for a kidnap victim in the United States, Mrs. Piper was found chained to a tree. Five years later, and only 16 days before the statute of limitations would have closed the book on the case, two men were indicted and subsequently convicted. The convictions were later reversed, however, and the two men, Kenneth Callahan and Donald Larson, were acquitted at a second trial in 1979. In Orono, Minn., on Monday of cancer.