VA Serious Mental Illness Act This bill requires the Department of Veterans Affairs (VA) to complete the development of a clinical practice guideline or guidelines for the treatment of serious mental illness. Under the bill, such guidelines must address the treatment of schizophrenia, schizoaffective disorder, and persistent mood disorder (including bipolar disorder I and II). The VA must carry out an explicit plan to implement and disseminate the clinical practice guideline or guidelines that are developed. The VA must establish the Serious Mental Illness Work Group with the Department of Defense and the Department of Health and Human Services to develop such clinical practice guideline or guidelines. Finally, the VA must complete an assessment of the 2016 Clinical Practice Guidelines for the Management of Major Depressive Disorders to determine if an update is necessary.