This is not really a full answer to you question but I think it would help if you further define your question and narrow it down a bit. You might first want to browse the internet and fully understand all the different uses of VPN’s to better phrase your question.
Are you talking about site-to-site VPN connections or client to site VPN connections? Are you using any routers/firewalls in your connection or are you intending to use the Linux box as the router/firewall?
Do some research and further define your question and then ask again.