Here is a basic explanation why the studies of Vietnam veterans were inconclusive.
In March 1990, the Centers for Disease Control and Prevention (CDC) released the results of the last of its studies. The investigators reported a 50- percent higher incidence of non-Hodgkin's lymphoma (NHL), a cancer of the immune system, among Vietnam veterans than among veterans who did not serve in Vietnam. However, the studies could not show that this increased incidence is related to exposure to Agent Orange. For example, Navy veterans who served on vessels off the coast of Vietnam tended to have a higher rate of NHL than did veterans based on land, and veterans who served in the region of heaviest Agent Orange use tended to have a somewhat lower incidence than veterans who served in other regions of Vietnam. The CDC could not determine why the Navy veterans had an increased incidence of NHL. No increased incidence was found for the other five cancers in the study (soft tissue and other sarcomas, Hodgkin's disease, and nasal, nasopharyngeal, and liver cancers).
In 1990, National Cancer Institute researchers reported the results of a study showing an increased risk of testicular tumors in military working dogs who served in Vietnam during the conflict there. Because the carcinogenic (cancer-causing) risk to dogs can be a useful indicator of carcinogenic risk to humans, another study was initiated to determine whether Vietnam service led to an increased risk of testicular cancer in humans. The results of this study showed a twofold increased risk of testicular cancer in Vietnam veterans. However, identification of specific factors, such as exposure to Agent Orange, could not be implicated as the cause of this increase.