The new gunboat arrived at Naval Amphibious Base Coronado, California, her home port, in July 1968. She conducted operational evaluation tests and participated in exercises with the U.S. Pacific Fleet. In addition, she conducted day and night operations with Fleet ships ranging from cruisers to conventional patrol craft. After a year of operating out of Coronado, "Tucumcari" was deployed to South Vietnam. She spent most of her six months in the combat zone assigned to Operation Market Time, the coastal patrol established to stop the flow of supplies from North Vietnam. While performing this duty, she logged 200 hours of foilborne operations including day and night, all-weather, and high sea state missions. She also conducted underway replenishments with larger Fleet units and vertical replenishment from helicopters. The latter included medical evacuation operations and the transfers of cargo and fuel.