HAVING REACHED a dead-end of government inaction, Vietnam veterans have now joined together to try to discover through their own resources what harm may have been done to American servicemen who were exposed to Agent Orange during the war. Under the Air Force's Operation Ranch Hand, Agent Orange -- a mixture of two widely used herbicides, 2,4-D and 2,4,5-T -- was sprayed over Vietnam in an attempt ot destroy its jungles and crops. Now the "ranch handlers" want to know whether Agent Orange affected more than the crops. So far, neither the Pentagon, HEW nor the Veterans Administration has been of any help.