Free tissue transfer in a previously operated neck with limited recipient-vessel availability remains a major reconstructive challenge and may require interposition vein grafts for venous outflow. The fate of these grafts after flap failure is poorly defined. We report a 77-year-old woman with recurrent squamous cell carcinoma of the right upper lip and malar region. Following oncologic resection, reconstruction was performed with a radial forearm free flap. Venous drainage was established using an interposition vein graft between the cephalic vein and the caudal stump of the external jugular vein. Arterial insufficiency developed within 24 h. Exploration on postoperative day 6 revealed arterial anastomotic thrombosis, while an Acland patency test on the first flap’s cephalic vein demonstrated backflow before the venous anastomosis was opened. The graft was retained and reused for venous drainage of a contralateral radial forearm free flap, which remained fully viable. This case demonstrates the technical feasibility and short-term functional success of reusing an interposition vein graft after confirmation of intraoperative patency.