WiMAX technology with up to 30 miles of coverage compared to other technologies is more vulnerable to jamming attacks, and therefore, the security issue was a major concern. The jamming attack affected the network by reducing throughput and increasing delay in different applications, several solutions and protocols were introduced for this issue. One of these solutions is a VPN that uses encryption algorithms for securing data transmission. Moreover, the network is integrated with the firewall to protect the server from any unauthorized access. Thus, the paper investigates the impact of VPN with the firewall for securing WiMAX using OPNET Modeler (v14.5) which was chosen as a suitable tool for simulating WiMAX networks. A VoIP application was applied with other applications in terms of sent/received traffic, delay and throughput. After running the OPNET simulation, the collected results showed that the existence of VPN with firewall increased throughput and decreased the delay which was caused by the jamming attack. The inclusion of the firewall will prevent any access to a specific application from the server but the VPN would allow access from a specific source (Base Station1) to access the server. The benefit of integrating VPN in the system is that the unauthorized client cannot access the server from any base station. This will provide more security for the system.