Network Function Virtualization (NFV) is an efficient approach to simplify and accelerate the deployment of diverse network services. A critical challenge lies in mapping Virtual Network Functions (VNFs) to high-volume servers, resource allocation, and traffic routing. In this paper, we study the joint problem of VNF placement on servers and traffic engineering for a network spanning multiple subnetworks. Each subnetwork is owned and controlled by a different administrator. We formulate the joint problem of routing and VNF placement cost minimization subject to flow demands where processing flows in local subnetworks is encouraged. To ensure sensitive information of administrators remains private and to cut the implementation cost, a scalable and decentralized approach based on the proximal Alternating Direction Method of Multipliers (ADMM) is proposed. Extensive numerical evaluations show the efficiency of our approach against existing work.