The VPN should be on a different subnet to the house network. Both house network and VPN will have their own gateways. Put the Ethernet/WAN on the media PC on both subnets (you'll need static IP addressing for both networks on this interface). Set the routing table on the PC to route iPlayer addresses (for example) via the VPN subnet and gateway (and ITV player etc.) and the rest via the house network and gateway. You'll need to set the routing tables for each machine you want to tunnel media. Depending on how your network and VPN are set up you might be able to do the routing in your router depending on its capabilities, in which case the routing can be made common to all machines on the network seamlessly.