Fonda's "My Life So Far," due in April, describes the French director as "cruel and misogynistic" for manipulating her into having sex with him and another woman at the same time.
"It seems shocking that I did that but I convinced myself that it was fine even though it was killing my heart," says Fonda, 67.
The book confirms widespread rumors the couple engaged in orgies on numerous occasions.
They were married from 1967 to 1973 and had one child, Vanessa. Vadim, a noted director, died in 2000.