VPN Switch office network

My Office is changing locations and we have decided to upgrade our network, 100 wired ethernet connections terminate into punch blocks and then are connected to our 4 stackable switches(I have not bought yet) which is then connected to our server, we have a DSL modem connection coming into our switch. this configuration needs to have a VPN added to it(for mobile users – I probably only need 5 to 10 Connections over the VPN at one time),
Does anybody know what the best configuration is?
I have looked for 24-port VPN Switches but the most I’ve found are 8-port – how would an 8-port VPN fit into my Configuration (between which hardware does it go)?
Thank you for any information you have