The demand for virtual private networks (VPNs) that provide confidentiality, integrity, and authenticity of communications is growing every year. IPsec is one of the oldest and most widely used VPN protocols, implemented between the internet protocol (IP) layer and the data link layer of the Linux kernel. This implementation method, known as bump-in-the-stack, has the advantage of being able to transparently apply IPsec to traffic without changing the application. However, its throughput efficiency (Gbps/core) is worse than regular Linux communication. Therefore, we chose the bump-in-the-wire (BITW) architecture, which handles IPsec in hardware separate from the host. Our proposed BITW architecture consists of inline cryptographic accelerators implemented in field-programmable gate arrays and a programmable switch that connects multiple such accelerators. A VPN gateway implemented with our architecture is transparent and improves the throughput efficiency by 3.51 times and power efficiency by 3.40 times over a VPN gateway implemented in the Linux kernel. It also demonstrates excellent scalability, and has been confirmed to scale to a maximum of 386.24 Gbps per tunnel, exceeding state-of-the-art technology in maximum throughput and efficiency per tunnel. In multiple-tunnels use cases, the proposed architecture improves the energy efficiency by 2.49 times.