Agent Orange Extension Act of 2015 This bill extends for two years the Department of Veterans Affairs (VA) presumption of service connection for diseases associated with exposure to certain herbicides, including Agent Orange, with respect to veterans who served in the Republic of Vietnam between January 9, 1962, and May 7, 1975. The Agent Orange Act of 1991 is amended to extend for two years the authority of the VA to enter into an agreement with the National Academy of Sciences to evaluate the scientific evidence regarding associations between diseases and exposure to dioxin and other chemical compounds in herbicides, including the association between exposure to a herbicide used in U.S. and allied military operations in the Republic of Vietnam during the Vietnam era and each disease suspected to be associated with such exposure.