How to configure browser to work with vpn?

Question

VPN

In general, there is a network with a regular modem provider through which vpn is passed. There is a user computer with Linux on board, which uses a router and a modem provider. On this user's computer, it is required to run a virtual box with a WinXP image, and it uses vpn and firefox walking through it.
Why exactly? Because I would not want to let absolutely all traffic through vpn, but only when there is a need for it.
Who will advise on setting up such a connection? Maybe you should not fence the garden with a virtual machine and can you somehow untie two traffic on Linux?

Answer 1

if only some specific port, and not completely the channel, then scatter through iptables.