Virtual Private Networks (VPNs) make use of public networks to emulate private networks. This technology gives a cost effective approach to linking up multiple remote communication nodes and forms a private network. This paper studies how to deploy VPN services in the mobile IP environment. Because of the mobility property, traditional VPN tunneling techniques can not be directly applied in the mobile IP based network. The VPN tunneling techniques are required to cooperate with the mobility offering mechanisms. The cooperation ways are not only one. In this paper, all the possible ways are proposed and compared. In addition, the fault tolerance and security scalability issues related to the VPN are also discussed based on the view of the mobile IP network.