The vision of 4G network is to have a unified network of heterogeneous wireless networks. To achieve seamless mobility in such a diverse environment, vertical hand off is still a challenging problem. Traditional handoff mechanisms were based on the Received Signal Strength (RSS), but to have an efficient handoff decision in a heterogeneous network environment in addition to RSS other parameters, e.g. monetary cost, user preferences and network conditions must be taken into account. In many situations unnecessary handoffs are triggered causing degradation of services, reduction in throughput and increase the blocking probability and packet loss. In this paper we propose real time services vertical handoff decision algorithm (RS-VDA). RS-VDA, defined for upward and downward vertical hand off scenarios. The proposed algorithm takes several metrics into account such as velocity of mobile terminal, bandwidth, RSS and delay of the network. The proposed algorithm will improve the overall efficiency of seamless mobility in Next Generation Networks (NGN).