In this work we propose a seamless vertical handoff solution. We realize the seamlessness by adding a virtual-interface module into current Linux kernel. Different from the mobile-IP based-vertical-handoff systems, there is no need of extra agents or handoff servers in our system. In the experiment, we show that the handoff latency is so short that it is unnoticeable to the applications. Rebuilding transport layer connections is also not required during the period of the handoff. Our system is simple and can be applied to current IP network. Therefore, it is instantly ready for real-world deployment.