Virtual private networks (VPNs) are widely recommended to protect otherwise insecure industrial control protocols. VPNs provide confidentiality, integrity and availability, and are often considered to be secure. However, implementation vulnerabilities and protocol flaws expose VPN weaknesses in many deployments. This paper uses a probabilistic model to evaluate and quantify the security of VPN configurations. Simulations of the VPN model are conducted to investigate the trade-offs and parameter dependence in various VPN configurations. The experimental results provide recommendations for securing VPN deployments in industrial control environments.