This paper presents the dynamic external Home Agent (x-HA) assignment, fast authentication, and pre-authentication in mobile Virtual Private Networks (VPNs). The proposed architecture is based on the mobile VPN proposed by the IETF, which adopts Mobile IP and IPsec. The IETF solution, however, leads to two questions: where should we put the x-HA and how should we trust the x-HA? We propose to assign the x-HA dynamically so the hand-off latency and end-to-end latency could be reduced significantly. By using Diameter Mobile IPv4 application, we also propose a technique such that the x-HA can be associated with the VPN securely. In addition, we also propose fast authentication and pre-authentication to further reduce hand-off delay. The proposed technique has been implemented in a mobile VPN testbed. Performance analysis based on empirical experiments is discussed.