A Virtual Private Network (VPN) is a means to allow you to connect to resources on the internet without making them publicly accessible.
In the context of embedded systems, it is often used for remote access: either for a command-line session, a remote GUI session, or to access a service running on the board.
Often, embedded hardware has dedicated cryptography accelerator units, such as the Cryptographic Accelerator and Assurance Module (CAAM) for some of the NXP i.MX series. On newer CPUs, the overhead of small workloads may cause the acceleration to have worse performance than using the CPU directly.
Also, hardware acceleration depends on the cryptographic algorithm being used by the VPN application. If it does not use one that is supported by the hardware unit, obviously there will be no acceleration.
It is not a guarantee that a VPN solution will use embedded hardware acceleration for cryptography, though it is possible that those solutions use the Linux kernel Crypto API, which will increase the likelihood of hardware acceleration.