Network Functions Virtualization (NFV) is transforming how networks are operated and network services delivered. The network is more flexible and adaptable. In particular, virtual Deep Packet Inspection (vDPI) engines can be dynamically deployed as software on commodity servers within NFV infrastructures for incremental monitoring. For a network operator, deploying a set of vDPIs over the network is a matter of finding the appropriate placement that meets the traffic management and operational cost constraints (license fees, network efficiency or power consumption). In this paper, we formulate the vDPI placement problem as a cost minimization problem. We cast the problem as a multi-commodity flow problem. We then propose a centrality-based greedy algorithm and assess its validity by comparing it with the ILP optimal solution on random networks.