PROMISES OF FREEDOM by David Grinstead (Crown Publishers: $19.95: 448 pp.). Can we continue to plumb the miseries of the Vietnam War and emerge with more truths? It's a formidable task that author Grinstead (an earlier novel, "The Earth Movers," was well-received) has taken on in this penetrating study of the impact that Vietnam had on the lives of five young Americans: one of the first American women correspondents covering the war; her lover, a mover and shaker in Lyndon Johnson's White House; two other lovers who become radicalized by Vietnam, and an Ivy League classmate who serves as a Marine officer. Flitting back and forth from stateside to the jungles of Asia, we see the idealism of the Kennedy years fade into the callousness of L.B.J.'s victory-at-all-costs pursuit of that futile war, and we see how these fresh young minds were twisted by it. The characters are three-dimensional and forceful, and we come to care very much what happens to them in this disturbing novel.