The evolution of mobile multimedia services outlines a trend towards the coexistence of wireless overlay networks. These networks have overlaid coverage areas and are composed of a hierarchical structure ranging from in-room to in-building, campus, metropolitan, and wide-areas. For these networks to work as a seamless inter-working, optimal mobility management has to be implemented. Beside the traditional horizontal handoffs between cells of the same network, vertical handoffs between different networks have more impact on system performances. One of the important challenges for seamless vertical mobility is to maintain a user active connectivity while the user is changing its point of attachment. In this paper we are addressing one aspect of the mobility management, namely the vertical handoff decision problem in wireless overlay networks, in this study, an algorithm is proposed to avoid the unnecessary vertical handoffs for such networks under different circumstances. Simulation results on the key parameters such as grade of service (GoS) and system utilization prove that the proposed strategy can provide powerful support for QoS in wireless overlay networks.