A PIECE OF MY HEARTPlay by Shirley Lauro, suggested by the book by Keith Walker. Directed by Allen R. Belknap. At the Union Square Theatre through Nov. 24. 'A PIECE of My Heart" is a discursive, impressionistic, and ultimately moving tribute to the women who served in Vietnam. Working from the book by Keith Walker, Shirley Lauro has created a dramatic montage in which six women contribute to a group portrait of sharply etched individual experiences. The action takes place on an all-purpose, multilevel-platform stage that serves the changing milieu and fluid treatment. After each of the characters has introduced and identified herself, Ms. Lauro assembles them on a transport plane bound for Vietnam. None of the passengers is more imposing than 18-year veteran Steele (Novella Nelson), a black intelligence officer whose tour of duty will include the most embittering experience of her long career. (When she comes to her superiors with advance information regarding the Tet Offensive, they refuse to believe her.) Martha (Annette Held), the "Army brat" who has become an Army nurse, serves initially as informal mentor. The group also includes Maryjo (Cynthia Carle), a guitar-playing vocalist of a USO-type trio; Sissy (Corliss Preston), a college graduate from Erie, Pa.; Whitney (Sharon Schlarth), a Red Cross volunteer; and Leeann (Kim Miyori), an Asian-American who has been serving as a Washington lobbyist for Vietnam vets. Tom Stechschulte stands in reliably for all the American men whose lives are touched by Laur o's women. Steele's rebuff is not the only disillusioning episode in "A Piece of My Heart." Vietnam proves a source of disillusionment for each of these women, just as it ultimately came to be for the nation that sent them. Steele summarizes the play's bitter indictment with: "Beirut was another Vietnam." Lauro's mordant critique follows its characters after their return home. Maryjo finds that she isn't going to be paid her promised salary (only her airfare). Whitney becomes an alcoholic. Sissy confesses that she watches "M. A. S. H." on television and cries. Steele quits the service. There are group therapy sessions for survivors who still experience waking nightmares. The drama's final, poignant moments occur at the Vietnam War Memorial in Washington, vividly suggested by designers James Fenhagen and Richard Winkler with the projection of a section of names of the 58,000 casualties. This vignette completes a deeply motivated theatrical collaboration whose individual contributions have been impressively coordinated in the production staged by Allen R. Belknap. "A Piece of My Heart" opens the Manhattan Theatre Club's 20th season and is presented by special arrangement with Roger L. Stevens. The play was originally produced by the Philadelphia Festival Theatre for New Plays and by Actors Theatre of Louisville as part of the 1991 Humana Festival of New American Plays.