Fifth generation (5G) wireless networksare characterized by applying the software defined networking (SDN) and network function virtualization (NFV) concepts to provide higher reliability and scalability, and lower latency for advent data-hungry services. In the initial version of SDN based 5G, a centralized architecture for SDN controller has been proposed, which inherently cannot fulfill those requirements simultaneously. This observation has led us toward distributed architectures, where multiple SDN controllers across the network constitute the control layers. Furthermore, to fully achieve the network flexibility and agility offered by SDN, and also improve resource utilization and consequently to reduce network cost, we incorporate virtualized SDN (vSDN) controllers, enabled through the NFV technology. In vSDN with distributed architecture, controller placement is one of the main challenges, which is an NP-hard combinatorial optimization problem with conflicting objectives: 1) maximizing the reliability of network with low latency; 2) minimizing the total cost of implementation, which can be interpreted as reducing the number of vSDN controllers. In order to achieve such a compromise, in this work, we formulate the problem of vSDN controller placement and introduce some performance metrics to model the latency of communication between controller-switch pairs and also robustness against vSDN controller failures in a network. The introduced formulation is some sorts of submodular optimization, which is exploited to develop several algorithms. Simulations results demonstrate that, with a negligible performance loss, a significant reduction in the number of vSDN controllers is achievable. Finally, latency, cost, and robustness trade-offs are investigated.