The Softether VPN has a different approach. It is able to connect to traditional hardware VPN's as Cisco, but it is also able to just use simple and effective SSL VPN between two PC's. This means that you will be able to set up a VPN connection between two offices using just any internet connection.
You don't need to use a raspberry for this, as the Softether VPN do run on most OS's and most VPN clients like smartphones can be set up to connect.
You can have a Windows machine on one site, Linux on another and both connecting to a Cisco at HQ. After that you can have a piVPN in bridge mode at home feeding you IP addresses from the office DHCP server.