Fiction: D.C. mood during Vietnam; In the City of Fear, by Ward Just. New York: The Viking Press. 291 pp. $14.95.

The Vietnam war, the central American tragedy of the '60s and early '70s, is no longer a recent memory. A generation has come of age that knows little of Ho Chi Minh, Quang Tri, or the Christmas bombings in Laos. For them, the past must be re-created.

The novels that sprang from the white heat of events have been written. The best ''report from the front'' may have been Michael Herr's ''Dispatches.'' Robert Stone's ''Dog Soldiers'' showed how the war came home, bringing drugs and chaos with it.

Now, Ward Just, an able, intelligent journalist, has made a sincere attempt to bring to life the moral confusion in Washington during the '60s. He focuses on an ambitious but reflective congressman, Piatt Warden, his attractive wife, Marina, and Sam Joyce, an Army colonel who is lover to Marina but is married to the war. They are the center of a group of Washington movers and shakers - lawyers, journalists, politicians, CIA types, who talk about the war, worry about it, and conduct its business.

But the characters who might be vivid remain curiously offstage. Dennis McDonough, a member of the White House staff, who wrote a courageous memo opposing the war, never makes an appearance. He dies in an unexplained auto accident, existing only as a point of reference for his friends. And Sol Henderson, the soldier who lives the craziness and violence of Vietnam, appears most convincingly at a remove - on a television screen.

Of course, this is Just's point. Those at home are detached - mired in their professional middle-class guilt, compounded of halfhearted liberalism, quid pro quo, and accepted infidelities. But there is a muddy weariness to the novel, perhaps best expressed in the words of Sheila Dennahay, a Langley employee whose son is in Vietnam: ''Poor, poor Shakerville, tired of the war, exhausted with the war, fighting it all day in the office and then again at home and even at someone else's house. Stuffing ourselves on it, we're fat as geese with reports from the war.''

While the novel abounds in details that portray the world of power - Sam Joyce's visit to the President, an elegant and chilling duck shoot with the President's adviser, the slick machinations of the lawyer, Henry Costello - it remains unsatisfying as fiction. Perhaps the theme of inaction and ambiguity explains the lack of real plot and the peculiar structure of the book, which presents as a last scene the funeral of Dennis McDonough, who has already died at the start of the novel.

Those who do remember the war will recognize familiar landmarks and concepts, even ''the light at the end of the tunnel.'' But ''In the City of Fear'' just may not hold the attention of readers who require fresh instruction in the light of recent history.