You have a couple of options. The most secure and efficient is to use the FVS114 as the VPN server/endpoint, but you have to purchase licenses/software, for the VPN clients. You need the ProSafe VPN client:
VPN Client Software - NETGEAR.com
If you were to purchase that we can help you with the configuration of see the following site:
VPN Case Study Site
The other option is to configure the Vista computer to be the VPN server:
Vista VPN Server
And configure the client:
XP VPN client:
WindowsXP VPN Client
Vista VPN client:
Vista VPN Client
and the router will need to forward port 1723 and enable GRE pass through. See the following for forwarding port 1723, however forward the built-in PPTP service rather than port 1723, and this will enable GRE as well:
Port Forwarding for the Netgear FVS114