“The Vagrants” begins on March 21, 1979 — the spring equinox — which is this careful writer’s way of telling us that a long winter of privation and darkness may be giving way, at last, to the blossomings of spring. It is set in one of the new nowhere towns of Mao Zedong’s China, 700 miles from Beijing, a bare, rationed place of small factories and overcrowded shacks laid out in anonymous rows. Eighty thousand people live in Muddy River, essentially migrants from the countryside, and, almost in the manner of a documentary filmmaker, shooting in black and white, Li homes in on a few typical souls whose names alone give you something of the settlement’s flavor: Old Hua, Teacher Gu, a dog called Ear, a deformed 12-year-old girl called Nini and a teenage boy as brutish and unassimilated as the name he brandishes, “Bashi.” All are victims of a crippled society that has effectively outlawed humanity and made innocence a crime.