VPN Device

Hello,

I am in the planning stage of a VPN implementation solution, and I have
chosen to do a hardware solution type. The current network flow is

         Users -------
Switches--------Firewall--------Router-------Internet.

I want to know at what point in the network flow will be the best to place
the VPN device without compromising data security, and at the same time
achieve minimal reduction in transmission performance.

Thanks for your help

Charles